My 98 Firebird (V6) has recently started making a soft 'burbling sound' which changes in line with the engine revs. When idling, it's a fairly soft noise, the pitch and speed rises as the engine revs. It sounds as if it's coming from the right hand side of the car. Initially, the sound was intermittent, becoming more frequent until it's now always there. Any ideas what it could be?
Cheers Dan