993 air con

please can someone tell me if my air con when started should run a fan for 5 sec. every 5 sec. constantly? ( by the sound of it a rather large one).
I have had new pressure switch/relay,but as yet not ventured into a possible Control-Unit switch as the cost is rather prohibitive (£1000)+ labour.
Any ideas?Anything I can try,I'm OK with electrics. regards KAJW.