Each and every single time I go to bed, my UPPER LEFT ARM near the inner bicep towards the armpit keeps THUMPING.

Thump, Thump, Thump........

The vein in there seems to be thumping and moving to my heartbeat for no apparent reason.

When I place my right hand in my left armpit, I can feel the vein shaking - or thumping.

Is this of any concern and/or what could be the problem?

Thank you.