Does any one Know if big ends knocking could be intermitant because i have a knock that sounds like it is at the back near the bottom of the engine, but it goes at higher revs and isnt always there . but it is most days. it seems to quiten down with the air con on but you can still hear it from underneath. Im not sure if its to do with my revs idling low because when up to temp it idles around 200rpm.
thanks in advance

 

Don't think the engines actually capable of sustaining itself at 200rpm! The correct idle should be around 750rpm. Don't forget that the rev counter dial only starts at 500rpm!
Other alternatives for the noise could be piston slap or cam belt tensioner. Needs to be listened to.
JohnD

 

if the noise is at the rear, it could very well be the turbo heat shield rattling. First thing I would check