For about 3 weeks now whenever I shut my driver or passenger door on my truck there is a rattling coming from in the dash on the passenger side, or when I am running down the Interstate and the truck shakes a little bit (tires out of balance) I can hear it also. It sounds like a ball in a can popping off the sides. I think it may be coming from the heater core? I really dont know. My heater core is not going out (or doesnt seem to be). Anybody have any ideas?

 

A mount may be cracked on that side or something maybe.
I know my dash rattles, but that's because it's cracked.

As far as it coming from the heater core, you could try burping the system to be sure.

 

Thanks for the quick response! I'll check it out.