2002 Vauxhall Astra noisey tick over

had subframe replaced and shortly after the engine developed a noisey idle which seems to disappear or lessen on acceleration. I swapped out the alternator thinking a bearing had gone in it as the knock/rattle is from that belt and pulleys side of the motor. no change so was wondering if a timing belt/water pump kit might sort it out as I need my car for work and hope that its not a serious engine problem. any help would be great, cheers