thumping

Okay so I have done a lot of work to this car and I have been able to solve a lot of the issues with it but this one is stumping me. There is a thumping sound coming from it, and this is what I have gathered. 1) Only starts when the engine fully heats up to the normal operating temperature. 2)...

So I had the bed loaded down with heavy equipment. After I unloaded I started hearing/feeling the thumping sound coming from underneath in the rear. The truck only has 1800 miles and should NOT be doing this. I looked underneath and everything "looks" ok the spare tire seems secure. Any...