It is possible that it is the reversing valve. You say that it is in the air handler though and the reversing valve is in the outdoor condenser in many heat pumps including Rheem and Ruuds. Rheem and Ruuds (same manufacturer) are different from other heat pumps as the energize the reversin valve in the heating mode instead of the cooling mode as 99% of other heat manufacturers do. It is possible this is what you are hearing.

If it is annoying and bothersome request from your Ruud heat pump dealer to send a qualified HVAC factory representative to investigate this issue. Good luck and i hope the problem gets solved.