just serviced A/C, now it makes short hissing sound!

[N00b-un-2]

I live in Phoenix so having working A/C is a must. I was noticing that my AC wasn't blowing as cold as it should so I went and got a can of R134 to recharge. I wasn't surprised that my low side was reading about 10 PSI low (it should have been above 55psi but it was sitting just above 40psi). I added the whole can of refrigerant, but the pressure never increased. Now my AC only blows cold very intermittently and I've been noticing a short hissing sound every so often when the compressor is trying to engage.
My thoughts are that I may have over serviced the system and now the compressor is not able to engage, or the compressor was going bad and the added refrigerant finally just pushed it over the edge.

Now before anyone tells me that it's a vacuum line or anything ridiculous like that, it is not a CONSTANT hissing while the AC is on. it is an intermittent hissing sound that lasts roughly a 1/4 to a 1/2 a second and happens repeatedly every 5-10 seconds while the AC compressor is trying to engage. It's as if the compressor is siezed and the sound is the pressure escaping from a bad fitting or something.

is this something I can fix or should I just take it to a mechanic?