Penguin 330 rattling noise

I Have two Penguin 330 filters on a 55-gallon tank. The one 330 is about 4 years old and the other one is two years old. I have been very happy with the performance of the filters - easy to maintain. The filters are fairly quiet because I maintain the water level in the tank. I did notice that the 4-year old filter has started to make a muffled rattling noise while it is running (not the loud noise associated with the priming operation). The impeller seems fine.

Any ideas about what it could be? Do I need to look at replacing the impeller?

Thanks!

Thanks for the reply. The intake tube could be the problem. I have had some problems in the past getting the intake tube seated properly.