I.ve recently noticed a thumping noise coming from behind the heater/AC control area. It stops when recirc. button is on
41,000 miles. Occurs intermittently, more often than not. Depressing the air recirculation button stops the thumping. When the air recirculation button is off the thumping returns, sometimes immediately, somtimes a short time later. No check engine light or other warnings accompany the thumping.