It's your innovatek reservoir. (From the picture you have the old style milled-from aluminum model)
No kidding.
I had a number (three? four?) systems built with these and they all cavitated a bit. You can stick a bit of open celled foam in there (the material eheim supplies as an intake filter for their submerged fish pumps works great) or a bit or rolled up nylon screening (use a heatgun or some such to fuse the exposed ends so you don't end up with a loose screen "wire" loose in your system).
Or you could just go for an updated innovatek reservoir. They're made of ABS, but they're alco solved the cavitation issue. The problem there (for your setup) is that the inlet is moved to the
top.
Oh -
high speed PC's got 'em back in stock.
