Strange TF2 sound issue

[Turtles]

I'm having a peculiar sound issue with Team Fortress 2. The sound changes while I'm playing, I can't specifically describe it, It just sounds different...It's like someone changing your MP3 player to Pop setting instead of Rock or whatever. It's really weird and I was wondering If you had any ideas. I've got it set in TF2 as Headphones, which is what I'm using. My Realtek HD Audio on-board sound seems OK. And my headset doesn't have any problems with anything else

Any help much appreciated

 

[Turtles]

It normally starts off ok, but then changes and stays like that until I restart TF2. It's not really a hum...just a change in the sound. Can't explain it