After a year or so of use I'm finally having a problem with my HW-302. The impeller is rattling so loud i have to wrap the filter with blankets to muffle the noise. I believe the problem is with the cap holding the impeller in place, it only has two tabs that lock down allowing the whole cap to tilt to one side or the other as you tighten it down. My o-ring has a crease in it now and i cant get it to set flat. So far I'm not having any luck finding a replacement cap, o-ring, and impeller. I'll have to e-mail a few people on e-bay that sell the filters and see if i can get parts from one of them.
you might be able to try the Aquatop filter parts too, it seems to be the same filter. I just ordered the CF-400 from trueaqua.