I've posted about this problem previously but didn't resolve it so i thought i'd try again
Basically after driving the car (2001 Astra SRi 2.2), for say 10 minutes or so, when it is at a standstill the exhaust will start to vibrate/ knock on the underside of the car. Its as though there is pressure or something from the engine thats making it vibrate, if i hold the revs at around 1500rpm it stops however when revving the engine it feels as though the car is going to stall, kind of splutters when i press the accelerator. If i turn the ignition off then fire her back up again the car splutters into life and there is a loud banging from the exhaust as if either a lot of backpressure has been forced out from the engine or something has ignited. This happens every time i drive the car, i can tell when its about to start doing it as the exhaust note changes to a very very deep burble when accelerating and when changing gear i can hear the exhaust vibrate/ knock on the underside of the car.
After letting her idle for a while this will all stop and will be fine on start up but comes back once she's been driven for ten mins or so.
I've had it into my local garage and they couldn't fathom out what was wrong with it, to be honest i did tell them to take her out for a good drive which they didn't do so this is probably why! They did mention that it couldn't be any form of 'backpressure' and could be a sticking valve
I haven't got a clue what it could be and i doubt it could be a sticking valve.
Has anyone got any ideas what's causing this or failing that does anyone know of a shit hot garage/ mechanic in the North West (preferably local to Warrington) that i could take it to. My car is quite heavily modified so i'd prefer not to take it Vaux.