I remember reading up on this when I was building mine. I think there are two options:
1) change/modify the path the water takes (use a rounded 90 PVC)
2) change the flow of water that travels through the standpipe. (increase/decrease flow)
Knowing this tank, I think I would first change the 90 under the tank (just after the bulkhead fitting) to help smooth out the ride the water takes. I could imagine the water going down the 3' and crashing into the 90 at the bottom. I found a smoother 90 and thats what I found worked best for me.
Another option is decreasing or increasing the flow. Pinch the hose on the return line. Does it get louder? What happens when you stick your arm in the tank? If I remember on that tank, my arm would displace water and it would get quiet for a few seconds. Probably what would be ideal is to increase the flow which would help overall water turnover rate.
Dana