HD6970 fan noise

I have a Diamond Radeon HD6970 card, non-reference, so I have two fans. It seems to be working perfectly whether stock or in overdrive. But, I have been getting a fan noise lately that I can't explain.

It is a quiet pulsing about 2-3 cycles per second. I can only hear it when all my case fans are at low rpm. I think most people wouldn't even hear it over my other fans, but I do. It is noticeable at GPU fan speeds of 20-30%, but I don't detect it over that speed -- don't know if that is due to fan speed changing or the fans are louder at higher rpms and I just can't hear it anymore. I don't recall hearing the sound when the card was new (about 45 days ago).

The sound is similar to what one hears in a bedroom ceiling fan that isn't perfectly balanced -- whoosh, whoosh, whoosh. Since the sound doesn't relate directly to the 1000+ rpm of the fans, could it be some aerodynamic harmonics that are set up between the two fans? I can tell, however, the sound comes from just one fan. If I place my finger lightly on the center of the fan while it is spinning, the sound stops.

Second question, I have never disassembled a GPU card before. Any challenges with popping a fan out and replacing it if I can get an RMA fan from Diamond rather than having to replace the whole card? I am thinking I would rather have them send me a fan rather than lose my card for a week or two or more before getting a card replaced.

Maybe its two fans running at speeds that when put together and clash. Its like two pitches that are really close to each other. It does that on my violin when i play two notes extremely close in pitch, but for you the pitches are a bit apart.
The only solution is change one of the fan speeds.

It's a side affect of PWM fans at very low RPM. My video card and CPU fan do the same thing. Sorta a grinding whoosh?

I have my card set in the BIOS to not even turn on the fan until 50C, and then at that point, it comes on at 45% to avoid that. I'd rather hear the fan spinning fast than that whooshing sound.

Thanks Crapxxas119. I know what you mean about the harmonics that get set up on two close pitches. I thought it might be that. I'm also a musician.

Thanks CompuTamer. Good suggestion on the BIOS temp setting. I agree, I can deal with a continuous white noise easier than a constant background pulsing whoosh.