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?