I had the same problem. I did a little research and ended up following the instructions pasted below. It helped, though still not perfect. The hissing/buzzing was driving me crazy enough that I was ready to return the phone, which I absolutely love otherwise.
I included the link to the actual post, but I don't know if I inserted it correctly. If the link doesn't work just search the Thunderbolt forum for "hissing during call."
I hope this helps.
Dial ##7764726. (Spells program) Go to "Nam cdma setting" Scroll down to "preferred voice service" (there's 3 entries) Change them all to evrc-b Hit menu and commit changes.
http://forum.androidcentral.com/htc-thunderbolt/74373-finally-got-rid-hissing-during-call.html