I just purchased a second Sapphire 7870 OC and crossfired it (which produced wonderful results) but when my PC is standing vertical, the fans on the card make a really bad knocking sound when the on-screen graphics start getting intense (Bioshock 3 and Skyrim so far).
I tried making sure the screws were down tight into the case, but it didn't make much difference. Any suggestions?
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I took the panel off my PC with Bioshock loaded up, and sure enough it was the PSU making all the racket. On a hunch I flipped it around so the PSU fan was facing up instead of down. It seems to stop the noise but also reduce the temperature on the cards by about 10 degrees Celcius.
I guess that settles the "top or bottom" PSU debate....