Under bonnet noises

170,000 miles, April 2016

For quite a few years my Mk V had a very annoying rattle. It seemed to come from somewhere under the bonnet. It only appeared when the engine revs were between about 1,200 and 1,400 rpm. This corresponded to me driving at 30mph in 4th gear and so I would often hear it when driving around town. It used to drive me mad!

I tried several times to find where the noise was coming from and the following were suspected:

All these parts felt quite loose-fitting and I imagined that they could start vibrating generating the noise I heard. I tried removing and then refitting each to ensure that they were fit correctly. I even placed a 1 inch thick piece of polystyrene on top of the battery to hopefully prevent the lid moving. But I still had the annoying rattle until I found the real source.

Air intake rattle source
Loosely fitting box (arrowed)
with lid removed.
Click on photo to enlarge.
Air intake rattle fix
Superglue applied to highlighted locations
to prevent rattle.
Click on photo to enlarge.

It turned out to be a box just in front of the air filter housing which channels air passing through the front grille into the air intake ductwork. This box comprises a body made out of two parts and a lid which clicks in place. The problem was that the two parts of the body were loose fitting and it was these that were vibrating and causing the noise I heard. To check I just packed where the two parts joined with a few small pieces of thin cardboard. And that was it!

It didn't take me long to confirm that this had fixed the problem. I could hear the difference straight away. I suspected that the cardboard fix would not last forever and so to hopefully fix it once and for all I applied superglue. That was it - No more noise!

Drooping under-bonnet sound insulation

Under-bonnet sound insulation droop fix
Under-bonnet insulation
held in place by plastic tubing.
Click on photo to enlarge.

Every time I looked under the bonnet I was a little disappointed that the under bonnet sound insulation was usually hanging down at the right hand side. I wasn't exactly sure how much sound this insulation was cutting out but I'd prefer it to stay in position.

It was clear what the problem was. The insulation had either shrunk or had lost some of its integrity so that the little tabs around its edge no longer remained slotted into the cut-outs in the bonnet. Searching the internet it was clear that this was a common problem. Some people had removed the insulation altogether. Other's had used all-sorts of fixings and clips to keep it in place.

The insulation is made out of a relatively soft, fibrous material and in my car, this was beginning to feel a little fragile and so I didn't want to mess around with it too much. I was a little worried that metal clips might damage either the insulation or the bonnet's paintwork . So after a little thought and seeing what materials I had at hand, I came across a length of clear plastic tubing about 1cm in diameter. I realised that I could wedge the tabs around the edge of the insulation into the bonnet cut-outs with a few 1inch long lengths of this tubing. So that's what I did. I don't think it looks so bad and the insulation has not dropped down since!

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