Engine: S60: B5254T2 chugging noise at idle

[ilmiont]

As some of you likely to see this will know, I've been away in Scotland for two weeks and in the middle of last week I wrote on these forums that I hoped I wasn't speaking too soon, but everything was running brilliantly. Well of course I spoke too soon

Towards the end of the week, a new noise appeared at idle. A quiet but deep chugging sound in the cabin, noticed initially when coming to a stop. With the driver's door open, it's clearly audible and seems to be coming from the belt area. It's easiest to hear with the bonnet closed.

Characteristics:

- sounds a little like a gently blowing exhaust
- only seems to be at or near idle but not completely sure
- the frequency fluctuates independently of the engine speed
- abruptly stops and goes silent; reappears upon blipping the throttle and returning to idle

In this video, the bonnet is closed (it was raining). The previous day I'd opened the bonnet (but had no camera to hand) and the aux belt and pulleys looked to be moving normally from a brief visual inspection. I put the camera where it was because it gave the clearest view of the noise.

https://youtu.be/FV33tRl6v1Y

- 0 - 7s: Idling without the noise. Blip the throttle and it does not appear.
- 8s - Blipping the throttle causes noise to appear.
- 9s - AC compressor audibly cuts in.
- 25s - Noise speeds up. No change in revs.
- 33s - Noise slows then stops at 37s.

Blipped the throttle again after cutting the video. Noise reappeared for a similar duration.

Timing belt gear done about 20k miles ago.

Aux belt and tensioner 10k miles ago.

Everything's running normally and it's developed over the past 1,500 miles or so. (Covered 3,600 miles in the past fortnight - I did not notice the noise at all in the first week.) Got back home again last night (450 more miles after taking this video) and it was very clearly audible as I was parking up; if you were passing by you'd notice it and think the exhaust was lightly blowing, that's what it sounds like to me with doors closed.

[ilmiont]

Bumping this - surprised not to have any replies, maybe the title is confusing but I'm not sure how best to describe it.

I've shot another video, this one on my iPhone unfortunately and the mic isn't quite so good, but still plainly audible.

+ YouTube Video

+ YouTube Video

I've noted more characteristics:

- Coming from somewhere low down on the off-side. Easiest to hear in the footwell (with door open) or next to the offside wheel arch. Can't really be heard from the top of the engine.
- It starts off quietly, gets considerably louder and slightly faster paced, then quite abruptly cuts off entirely. After it cuts off, it will come back again, usually either by blipping the throttle a few times or waiting a minute or so.
- The belts and pulleys look OK to me.
- Turning air con on or off has no immediate effect.

I asked my dad to listen to it and he independently described it as "like an air noise" or a "belt fluttering" which is also my impression really, I've said before it's a bit like a blowing exhaust, like air is being expelled by something.

It's making the noise at idle throughout the new video. At the start of the video it's just started and is fairly quiet; later on from 00:26 when I return to the offside wheel arch and then look under the car, it's very noticeable. Also when I go to the footwell.

No clues in VIDA. Car's driving completely normally. It started doing it while we were away Scotland. We've covered 1,500 miles since it began... everything's still intact, but this definitely isn't normal and it wasn't there when I went away!

I'm stumped and would appreciate any ideas thanks. What's in the area of the front offside, quite low down, that could make a noise like this, that spontaneously starts and stops? I was suspecting air con but it keeps going with air con turned off.

Also of note - compare the two videos above, the top one is the new one, 450 miles after the bottom one. Both videos at idle ~650 rpm. The noise in the new video is consistently faster than the noise in the earlier one! The car was cold on the driveway today and fully hot in the earlier video.

Something's definitely out of the ordinary somewhere

[Joe Harding]

Vacuum leak. Lift the oil filler cap and it should be much louder. Could be engine mounts be leaking? CheshireD5 will give you all the info you need to know, or YouTube SiRobb and see if he has a movie.

[ilmiont]

Thanks I'll try lifting oil cap and see what happens. I didn't think the petrols have vacuum engine mounts though?

I'm bemused as the car's running fine and has been like this for close to 2k miles now. Holds a steady idle, seems to run as responsively as ever, no DTCs, only this noise which starts, accelerates, slows, and stops of its own accord.

[marcb]

I have the same engine in a 2005 V70. Have to say that unusual noises are not uncommon and come and go and I tend to ignore them unless something is obviously wrong. As long as the fluid levels are all OK...

Yes, there are no vacuum mounts on these.

If I were to look at anything it would be the pulleys on the alternator and steering pump and the pump itself. Some things are related to temperature I reckon.

[ilmiont]

I've found this car now which does sound similar (described by them, perhaps more aptly, as a "chuff"), has similar characteristics with no stated performance loss, and was a vacuum line:

https://youtu.be/r9B0CiirP54?t=280

I'll try and inspect all the hoses I can see more closely later in the week. I'm still waiting to hopefully get better parking soon which would give me more opportunity to investigate things.

[ilmiont]

Problem resolved - this was a disconnected EVAP hose, located slightly inboard of the offside front subframe mount:

https://www.volvoforums.org.uk/showthread.php?t=321436