we have a n reg 1.6 16v astra and its making a making a knocking / grinding noise when you start the engine !!! its coming from the side where the battery is ? it only does this when the engine is cooled right down . once you have been driving for about a minuite it stops and wont make the nosie untill the car has been left for over 2 hours to cool . can someone help as there seems to be nothing over that side of the engine just the battery ? pls help
Dunno about the Astra, but many Vauxhalls (I'm thinking Cavalier etc.) have a Secondary Air Pump located around where you suggest, that for the first minute on a cold engine blows air into the exhaust system - I believe to help warm the Cat up / subsequent improvements on emissions. Check the exhaust manifold, and see if there's a wide bore pipe (25mm+) heading down towards the area of the noise - if there is then that's where I'd guess the problem lies.
The location of the pump on a Cav, I forget (was 3+ years ago), but I think behind the wing/front grill and the wheel arch liner (a right bugger to get to)
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