Mr. Toad

I recently took my car (approximately 21,000 miles on the odometer) to the dealer for an intermittent noise at idle, which seems to be coming from the engine compartment. The noise tends to be more noticeable when the engine is relatively cold, but it does still occur when warm. When seated inside the car with the engine running and at a stop (e.g., stop sign or traffic light), it sounds rather like a helicopter approaching from a distance, with the sound fading in and getting progressively more noticeable, and then eventually fading out in complete cycles. The shop foreman opened the hood with the engine running and had me place my hand on top of the purge control valve, which seemed to thump in a relatively similar time sequence to the sound heard when inside the car with the doors closed. They replaced the purge control valve, and while the noise seems diminished, I can't say that it's entirely gone.

Has anyone else experienced this?