Check voltage of the car running and when It's off. A healthy battery should be somewhere around 12.6 dead I think 12.1. Running should be around 14.4, I havnt checked my car but i would be worried with anything lower than 13.5 volts.
And it is common sense that when heat is applied molecules expand, air metal, and yes rubber, expand with heat.
2003 GMC Sierra SLT 2500HD CC/SB