Getting to the bottom of the lumpy idle mystery
Changed the dump valve, no better I think, will give it a run and check tomorrow.
Anyway, mooching about under the bonnet and noticed a slight sound above the engine noise.
noseying about I located it to be a cyclical sound, ie 1/2 second on/off.
Its not that loud, and the bit that is making it is a sealed black casing behind the fan on the rad to the nearside of the engine bay, low down at the front.
Theres quite a lot of space their so Im guessing it would be where the air con goes (dont have it), its just behind this space (towards the cabin).
any ideas?
Andy

 

Cam belt tensioner ??

 

yep, suggest cam belt tensioner also,they can make quite a knock noise especially when engine cold.

 

So nothing to worry about then?

 

Whoa there! IF the sound you are hearing IS the cambelt tensioner, the noise is a problem. It could be a precursor to failure, and that could prove very expensive.

However, the cambelt tensioner is, unsurprisingly, inside the big black plastic belt cover. This "black plastic casing" you're looking at, what shape is it and where, exactly, it it? Are there any pipes/wires connected to it etc. etc.?