howling drum brakes

I have an intermittent howling sound coming from my drums. 8.25 axle, 4.10 gears, 31" tires. I have replaced the drums, shoes, hardware, and the axle bearings. Mostly i notice it when driving slowly (say a parking lot) and lightly applying brakes. Doesn't seem to be a pattern as to when it does it or not.
Any suggestions?
Thanks!

Did it start after you replaced the brakes and drums? I'm having the same problem on my 93 xj after replacing shoes and drums. I'm trying all new springs next.

If you say that the howling is only coming from one side of the brakes and they wore faster than the other side did you may want to check a couple of things. Make sure that you don't have a parking brake cable that is hanging up. You should be able to access it underneath easlily. Also make sure that the wheel cylinder moves in both direstions. Either one of those could cause the brake to hang up just enough to give you an intermittent "howl".