Weird noise when playing back vocal track with buffer size under 1000

[mixmasterc]

I have 9 channels of midi and 2 aux using Sampletank. When recording vocals I get a delay so I bumped my buffer size down to 500(can't remember the exact number). When playing back the vocal track there is a weird noise. It sounds like feedback of some sort.

AMD 1800
Asus a7m
512 mb ddr ram
2 80 gig WD HDD

AGP - ATI 7200
1 PCI - FAN
2 PCI - HDD Controller (promise)
3 PCI - Digi Card
4 PCI - Diamond Stealth Video Card
5 PCI - NIC

I don't think it has anything to do with my second video card as I am showing 1/3 PCI usage and very low system usage.

I don't hear any feedback in my headphones or my monitors when I am recording in 500 buffer size.

Any thoughts or ideas are greatly appreciated.

Thanks

[mixmasterc]

bump