Michelle,
This sounds VERY much the same as something i went through last year.I was stressed beyond anything i'd ever went through for 2 months straight.One night i layed down to go to sleep,and i got my first feel of it,i thought i was going to die.It seemed to bother me the most at night when i was aware of it,because it was so quiet.I went to the dr,i had all the tests you did,he assured me it was PVCs from the stress,and gave me some pills,i am not a pill person,and didn't take them.Those beats made me crazy,and couldn't be ignored at all! that thumping is something i just could not ignore.
After 2 months,things were better in my life,and kept getting better,the stress was starting to dwindle,and they went away! I was falling alseep with headphones on just so i wouldn't be up all night,and aware of those beats.I trusted the dr,and he was right,he told me not to let any other dr put me through hell with stress tests etc. after the halter monitor,he told me he was almost positive it was the stress.He also told me i could take Magnessium.
Hope your doing better,Krissy