I had exactly the same problem - the reason I ask about the constant sound loop.
Seems to be a problem with BC2 and sound drivers along with PunkBuster; follow this link Bad Company 2 Fix
Follow it to the letter and it should be fine.. I did what the link outlines and its been perfect since - Zero crashing. Another vote of success; my brother had the same problem. Now solved too..
Pop any questions in here if you need to and I'll keep an eye out.
Edit: you do have Realtek sound on your motherboard?
hmm well i uninstalled pb from steam via the uninstall exe in steam which stopped me from joining a game, so i tried to reinstall pb from pbinstall but that said it was already installed, so i had to find the pb folder in steam and deleted that, and then update which now lets me into a game but i still crash in game, i have just found that realtek have released new drivers today which i have just installed but not yet tried to see if its fixed my problem, if i continue to have problems i will format and reinstall everything tomorrow i think.