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MG MGB Technical - Unusual noise from my MGB

For the last few hundred miles I've been getting a strange 'swishing' noise from my BGT. It's fairly constant, about twice a second, and when I'm sat in the car it seems to be coming from the engine/transmission area. When I check under the bonnet (hood) I can't hear it, probably because of all the other usual engine noises. I can hear it when parked with the engine running, at any speed on the road, any gear or coasting in neutral, any road surface, and always at the same frequency - about twice a second. It only stops when I turn off the engine. The car is standard unmodified UK spec 1979 BGT.
Brian Shaw

Have I got this right. The noise frequency IS unrelated to either engine rpm or vehicle speed? If so it could be related to fuel flow/pump, a water/steam leak, vacuum leak, your pacemaker or a panting dog!
Allan Reeling

Allan - the frequency of the 'swishing' noise is always about twice per second, at any engine speed, any road speed. It's there from start-up, even with a cold engine so that would rule out any steam/overheating. I park in the same place at home, and there's been no sign of any water leak. The car runs well, and emissions were well within spec on the recent MOT. Would a vacuum leak give noticably poor performance? I'm wondering about the oil pressure relief valve, although the oil pressure gauge still indicates more or less the same as it has done for years.
Brian Shaw

Could it be the fan motor running at a reduced speed?
Nothging else would remain at the same speed.
darnoc31

There are a limited number of systems whose operation is isolated from engine/vehicle speed.
Heater fan, fuel pump, electric cooling fan, radio, clock, relays.
The latter can rhythmically cut in and out if they, or connections, are faulty. Faulty electric clocks make a strange noise, but you would have noticed if time stood still! Fuel pumps work at about that speed.
Then there's the dog!!
Allan Reeling

I had a very similar problem, it turned out to be the rear right wheel bearingss It sounded like it was coming from the front right engine bay area, but I couldn't find any problems. I was onn the road from LA to Seattle, and had several shops check the brakes and shocks, with nothing found. I read a thread about rear wheel bearing possibly being the problem. They "looked", good, but as soon as I changed them, the noise stopped and never came back.
Joe Walck

Here was my thread when the problem started:
http://www2.mgcars.org.uk/cgi-bin/gen5?runprog=mgbbs&access=&mode=archiveth&subject=71&subjectar=71&thread=2011072913101413719
I can't find the followup thread that says what all I fixed and changed before I got to the rear wheel bearings...which finally fixed it.
Joe Walck

Joe - the noise I'm getting can be heard when the car is at standstill (with engine running). It only goes away when I turn off the engine.
Brian Shaw

When does it start - with the ignition or only when the engine is started?

If the latter how soon after it has started?
PaulH Solihull

Paul - the noise is there only when the engine is running. It isn't a loud noise, just perceptible above all the other usual MGB noise you get in the cockpit. It can become quite hypnotic when crusing at a steady speed for any length of time. It started about a year ago (= a few hundred miles for this car).

I thought I'd found something in the archive - Ross (Denmark) described what I'm hearing - a continuous sh-sh-sh noise, but his turned out to be shims missing from the front wheel bearings.
Brian Shaw

Maybe the turn signal or hazard flasher is buzzing? That could be about 2x per second, and independent of any spped etc. But you would think that the noise would be there if the ignition key is in the run position and the engine is off. Simple to test by unplugging them.

Ralph
L Poupard

Sorry to butt in but I had something very similar occur with an A Series engine many years ago. I eventually realised the oil pressure gauge was moving slightly up and down in sympathy with the Sh Sh Sh noise. I guess it was a noisey oil relief valve.
Robert (Bob) Midget Turbo

"shims missing from the front wheel bearings."

And surely changing with road speed. Similarly I'd expect any noise from the oil pressure relief valve to change with engine temperature and revs.

I can't think of anything that only occurs with the engine running, but doesn't change with revs or road speed - on a standard car anyway.

It's got to be pretty loud if you can hear it above all the other noise, I can't hear my fuel pump with the roof down, and not at all when driving.

As a matter of interest, what happens if you get up to say 30 then switch off the engine but leave it spinning from forward motion of the car? And what happens if you do the same but knock it out of gear?
PaulH Solihull

Paul -

Just been for a drive and this happened:

"... what happens if you get up to say 30 then switch off the engine but leave it spinning from forward motion of the car?" - the noise stopped when I switched the engine off.

"... And what happens if you do the same but knock it out of gear?" - cruising at about 30, put the car out of gear, noise is still there; then depressed clutch pedal, noise still there; the noise only stops when engine is switched off.

It isn't all that loud, I think it's noticeable because of its high-pitched frequency being different to the other sounds from the engine etc. It does seem slightly louder when the engine's under load.

I had another listen under the bonnet but it's impossible to hear it there.
Brian Shaw

I actually meant to switch the engine off again for the second test, the only difference between the two being that in the first the forward momentum of the car was mechanically spinning the engine, but the second not. But if the noise stops when the engine is switched off, but is still spinning, (and dousn't happen with just the ignition on as you have said previously) then it can only be something to do with combustion. I'd get it up on ramps and axle stands (or a garage lift) and listen underneath.
PaulH Solihull

Could it be the instrument voltage stabiliser?
Dave O'Neill2

OK Daft suggestion...When is a swish a tick? Fuel pump? Tho twice a second seems a bit fast. It would stop when you turned the engine off ( but ignition on) as the float chambers would be full....
Michael Beswick

Definitely not fuel pump, which makes the usual clicking noise when I first switch on the ignition. Sounds a bit like someone saying "sh-sh-sh-sh-sh....", like the sound of pressure building up and being released, about twice a second. I suspected it may be the oil pressure relief valve, which I have noticed oscillates at about twice a second on my other MG, but the oil pressure gauge on this car is nearly always steady when engine revs are steady (about 50 psi at 1000 rpm and 65 psi at about 2000 rpm and above).

Is it possible for a voltage stabiliser to make any noise (late MGB, presume it's solid state).
Brian Shaw

Brian, If it sounds like miniature Maracas, it's the gear in your Tacho. Mine (66) has that sound. I thought it was the speedo, but stationary with engine on it must be the tacho.
Can you hear it away from the cockpit?

Brian.
B. G. Griffin

Voltage stabiliser could tick (although I've never heard either of mine) but it would start within a couple of seconds of turning on the ignition i.e. with the engine still not started. All MGBs originally had the thermal stabiliser, solid-state is after-market.

Tacho is solid state with the exception of the movement, if it's not visibly moving at a constant engine speed I can't see it being that. Easy to check though, just pull the green fuse with the engine running.

If it still happens with the engine switched off but in gear and the forward momentum of the car still spinning it, then the oil pressure would have changed, but the noise apparently stops with the ignition off. I would like that double-check though, a noise that doesn't start with the ignition, only when the engine is running, but stops when the ignition is turned off but the engine is still spinning, sounds even less likely.
PaulH Solihull

This thread was discussed between 05/05/2012 and 14/05/2012

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