Attwood pump question

[The Revenge]

Just replaced my shower sump pump. It's an Attwood Sahara 500. It comes on and discharges normally. However when it shuts off it makes a small thumping noise. Starts out slow then speeds up and goes quiet. Lasts about 10-12 seconds. Then it comes back on normally when water starts to fill the plastic box. It makes the noise with or without the plastic lid on the sump box.
Any thoughts of ideas? I just don't remember the old one ever doing that.
Thanks