Weird rhythmic "thrumming" sound, and it's not Ravi Shankar.

[Pablothetrucker]

Hello all. 

My 1.4t SE has just developed a liw level rhythmic thrumming sound when at idle, accompanied by a sort of "trickling" sound. Both seem to be behind the dashboard and I can't hear either from outside the car or from the engine bay. The "thrumming" seems to stop when A/C is switched on, and "trickling" stops after about 10 seconds. I suspect an A/C issue even though A/C seems to work very well, but before I spend £132 just getting vauxhall to have a look at it, wondering if anybody has any similar experience and/or advice please? 

Many thanks

Paul (aka pablothetrucker)