"Droning" sound from engine

[nfearn]

One of the minor annoyances on my new 993 is a droning sounds from the engine which comes and goes but seems worse when the engine is cold. I've checked all the belts for tension and none seem excessive, so my thoughts are now turning to other possible causes.

Is there any history of noise caused by the oil pressure relief valve in these engines? My 944 suffered a similar drone for a while which I eventually traced to that valve - it was resonatiing in the near-closed position...

Any other thoughts as to possible causes would be gratefully received.

Nick

 

[geolab]

hello, pressure relief valve, there are two, and both are in the oil pump core, inside the engine block, no way you could hear those.
one opens at 5.3 bar and second does at 9 bar.
if the word droning is a bass type sound Humm, I could only think of one source, the A/C compressor
try to determine the area where the sound is coming from.
one simple test for bottom of engine, is sitting at the driver seat with engine revving, open the door fully, window shut, and close it very very slowly.
The door acts as a perfect noise deflector, no kidding.
At one point, you could clearly hear the valves functioning. Check if you hear the hum.

 

[nfearn]

I'll certainly try that and let you know. The aircon compressor was my first thought (the belt felt very tight to me) but having slackened it off the noise is still present. That however doesn't remove the possibility that the bearings are already shot.

Thanks for the info.

Nick

 

[Porker993]

Nick, easy test for aircon compressor bearing rumble, turn the aircon off at idle, noise should stop. Mine does ! It also goes away when the aircon has been used properly, and the bearings have been properly lubricated. That would be my guess.

 

[nfearn]

Mark, hi

I've tried this but it doesn't affect the noise. However, looking at the compressor, there's one pulley which is rotating all of the time and one which rotates only when the aircon is switched on. There's some kind of clutch between the two.

My theory, such as it is, is that the pulley that's driven all of the time is the one making the noise. The only sure way to eliminate or confirm this is to remove the belt, which is my next step in the diagnostic process.

regards
Nick

 

[38F3E1]

Yes the front part of the air con pump is rotating all the time. The rest only rotates when the magnetic clutch engages when you turn air con on. Taking the belt off is the only way to check it. Mine squeaked rather than droned - had bearing replaced and still squeaks so currently scratching my head over it.

The other culprit for your droning noise might be the power steering pump. They can make a drony grindy sound if the fluid is too low or if it has been too low before and is damaged.

Ian.

 

[nfearn]

Update on my investigations:

The aircon pump is definitely not the culprit - I removed the belt driving it and there's no difference.

I've also flushed the PAS system through with new fluid and that's made no difference either.

So, the investigations continue. I'm beginning to wonder if it's something like a resonating heat shield or similar. The noise is only present at low engine revs (say, 1,000 to 1,500 rpm) so it may be something along those lines.

Don't get me wrong - this isn't a banshee wail or anything like it, just a sporadic low drone that I think is not right. Performance, oil and fuel consumption all appear normal.

regards
Nick

 

[Porker993]

I suppose engine tin buzz is a possibility, especially if you've had engine work done recently. I found the CAT bracket on mine had half snapped off, this is a common problem, apparently.

Problem is, there is so much metal back there, with heat shields, pipes, joints, bumper stays and engine tinware galore, the only answer is to take it all off, fix, tighten, replace and clean everything where necessary and put it all back again. Now, I am remarkably buzz free for the first time in 8 years of ownership. Turned out my major buzz was from the rear PU tin, which is kind of the outermost layer of protection.

Time consuming, but not too difficult and strangely satisfying. All my exhaust bolts needed cutting and replacing though.

 

[clubsport]

If they are not recent, is it worth changing the belts, in case one is twisted or breaking down?

 

[nfearn]

The belts look fairly fresh to me, so I think I'll leave them for the moment.

Engine heatshields (and allied bits and pieces) are a definite possibility, but I think I'll leave further investigations on this topic for the moment while I fit my new dampers. Gert has been in touch to say they should be with me this week sometime. Then it's off to Center Gravity to get the wheels pointing in the right direction.

regards
Nick

 

[Cater_Racer]

I have same noise problem, I was told it's the alternator bearings??

 

[nfearn]

It's possible, although I would expect duff bearings to present as a drone throughout the rev range, and this noise is limited to relatively low revs.

Does your whine keep going through the rev range?

Nick

 

[Johnny C]

Easy way to tell if it's the alternator bearings, turn the engine on from cold, let it run for a minute and turn the engine off. Then drip some water onto the pulley wheel, if it sizzles it's the bearing. I had a bearing go. I got dropped off at the arches near 9M as the tow truck couldn't get under. In the short drive round to the garage - 200yds?- after Robin jump-started the engine the wheel was red hot.

 

[Cater_Racer]

No mine is only at low revs, sound like iterference on the radio, like it wasn't tuned in. But definatley comming from the back of the car, much louder inside.

Could it be the radio???? No I don't think so, it seems to go away when warm.

 

[nfearn]

ORIGINAL: Johnny C

Easy way to tell if it's the alternator bearings, turn the engine on from cold, let it run for a minute and turn the engine off. Then drip some water onto the pulley wheel, if it sizzles it's the bearing. I had a bearing go. I got dropped off at the arches near 9M as the tow truck couldn't get under. In the short drive round to the garage - 200yds?- after Robin jump-started the engine the wheel was red hot.

That's an interesting thought and I've just tried this test. Result: the pulley wasn't even warm to the touch.

The noise is only audible inside the car and not from the rear, even with the engine cover up. I'm inclining towards vibrating tinware.

Nick

 

[Cater_Racer]

Nick

I wonder if it could be radio interferance? We both have exactly the same noise, of that I am sure.

Gerry

 

[nfearn]

My noise occurs with the radio switched off though, which would argue it can't be that.

It definitely sounds mechanical to me, though not sufficiently identifiable to put my finger on it yet. Time will tell.

Nick

 

[Cater_Racer]

My radio is off too, but I just considered that the speakers wires could pick up strong electrical impulses from the ignition somehow??

I'll investigate further and report back.


Gerry