They hiss when they're getting low on gas (uncondensed refrigerant expanding
in the expansion valve). At some point the loss will mean the low pressure
switch will drop out, then you'll get nothing.

Your guess is probably correct. Most likely to be the condensor, or
corrosion on the pipes under the pipe clamps. You might find some obvious
corrosion, otherwise you need the proper equipment.

www.poolecool.co.uk

O-rings.  One is leaking and all the rest are at the point of failure.  It
may be economical in the long run to replace them all at this time.  Shops
often repair the worst or most easily detectable leak, and send you on your
way.

Earle