A/C making popping noise under dash

My a/c is acting very strange on my 1998 tacoma 4 cylinder. The compressor cycles about every 5 seconds and there is an audible pop under the dash that is in sync with the cycling of the compressor. I believe it is the expansion valve, but I'm not sure. The low side shows 55 psi and cycles down to about 40 when the compressor kicks on. i looked at the sight glass and there appears to be no movement across the glass. Any suggestions would be appreciated.