S40: Intermittent 'Thump' noise

[borzio]

As the title says, occasionally I'm getting a heavy 'thump' noise from somewhere directly in front of the driver. My car is a 1.6 2004 S40 32K on the clock.
When this happens, there is a jump forward motion instead of a smooth drive away, as if you are going to stall on a hill start. It's a heavy sounding noise, very noticeable but as I said, very intermittent, sometimes several times a day, then nothing for days.
The car drives perfectly all the time with no other issues. The clutch works ok and the gearbox is generally smooth.
I've had the car at the local garage and 3 mechanics drove the car separately: two never got the noise or the jump, one did hear the noise, but the car didn't lurch with him.

The mechanics took it back and checked all the joints and mounts, suspensions etc for tightness etc, but found nothing wrong. In fact, they could find no problem with the car at all.
They said to bring it back if the problem got worse and was happenening regularly so they could identify whereabouts the noise is coming from.

I've read somewhere (correct me if I'm talking tosh) that there is a torque converter/mount/something? that has this sort of problem noise, but don't know if that applies to my car, or if it would have different symptoms.

Any other suggestions would be appreciated.

[coffeejunky]

This will seem like a random question, but: Does this only happen with the aircon on?

Checking all engine mounts would be a first port of call, if any look split or perished, it is time to change. They are the rubber (usually cylindrical disc shapes of rubber attached to a meaty metal mount. A bit like a hockey puck).

[borzio]

Hi coffeejunky, no: I very rarely have the aircon on, most times the noise happens when it is off.

I had the mechanics check all the mounts etc but they are 100%, also there is no regularity to the noise happening.

Today I went 160 miles and it never happened at all, but it has happened for example doing a 3 point turn out of the driveway in the morning....just the once, then all is ok. Also today the gear change has felt a bit rough going down the gears on occasion, with a a rough rubbing feel through the gearstick. The gearbox oil and clutch fluid was changed a few months ago and the levels of the clutch and gearbox are fine.

[big_yin]

Just a hunch but could it be a starter motor problem?
Like a pinion not retracting ?

[borzio]

big_yin, thanks: no I don't think so (not that I've ever heard that noise), but more because it is random when it happens more often long after the car has been started.

The mechanics I quoted are very thorough, though not Volvo specialists, and even the one who heard the thump could not place where it came from, or find a cause when it was back on the ramp. It's wierd as the car drives perfectly otherwise.

[flyingtech55]

Have you looked in the boot? Sounds daft I know but not the first time forgotten (heavy) articles have been left in there.

HTH

TimR

[borzio]

flyingtec55, sorry for the delay in replying, though the delay has helped:

There is nothing in the boot of that nature to make a noise while loose, but this morning I had the 'thump' on turning right hard lock after reversing out the driveway and turning at low speed.
It was really a heavy noise and from almost dead centre ahead of the dashboard area, in the engine compartment, ie not within the cabin.

The thump was only once, hasn't happened again today even though I've just come back from doing a 160 mile round trip with many stops,starts, turns etc.

All suggestions welcome,cheers

[big_yin]

Does it only happen when you turn?

[druid]

Tuppence worth -

Spring moving in a strut? if it works out of place on full lock it has to twang back in - with a thump. Might take a few miles to happen.

[borzio]

Hi guys, sorry for the delay in replying:

big_yin, ....no. The thump happens most often on the hard lock, but the thump comes from the centre of the car forward of the radio area and can occur in straight line driving. It's a one-off noise that is driving me crazy. If the darn thing would repeat consistently ( say everytime I hard lock) it would make locating it easier. The only consistent thing about it is that it comes from the centre of the car as opposed to where the suspension is at either side.

druid...I'd love it to be that, but the location of the thump says no. I had a spring on the driver's side replaced a while back when a pothole snapped the thing, but the garage stripped down that coil spring when I took it to them for the thump just in case, and found nothing wrong with the fitting, and refitted as before. I'd expect a noise from there to be to that side?