Powerhead Placement (before or after filter?)

[redbouche]

I wonder if any of the fish experts here can school me on powerhead placement. I've got what is essentially a passive canister filter (aka no pump) that I'm using as supplimental biological filtration on my discus tank. I'm using a Maxi-jet 1200 to drive it. It's a closed loop system just like a canister filter would be. Right now I have ithe pump mounted after the filter, so it's sucking water out and then returning it to the tank. The flow rate is good, but it started silent a few days ago and has now developed a buzz and vibration. I assume this could be cavitation. I thought maybe my flow was too restricted, but it still vibrates when I pull all the media out so it is basically pumping through an empty cavity. So long story short, is there problem with locating your pump at the end of an inline set up rather than the beginning? Is there a mechanical reason why a pump would rather push than pull? Thanks for the help guys.