Noise in firewall
 

Starting last summer in Phoenix, a popping sound started, sounding like something like a wheel bearing. Was on the road and a mechanic checked it and found nothing loose or dangling. He said that the MPV had a problem with the welding in the wall between the engine and passenger compart. Which allowed it to expand in the heat. When fall came, the noise disappeared totally. Now, with the heat returning, the sound is returning. It sounds like something is wrong. Anyone had this problem?