Rumbling sound at spaced intervals from unit

by Mia
(Oklahoma)

When we run the air conditioning it makes a rumbling sound which can be heard in the garage. It's only every few minutes it does it but our house doesn't seem to be cooling either. The sound is a short rumble and then nothing for a few minutes.

I checked around the unit and the fan is moving fine and my husband thinks the sound is coming from the compressor.

Our ac unit is a Ruud zephyer I believe.