Help with noise please

I bought a 95 Honda Civic EX with 74k on it. I had a mechanic replace
the timing belt and 3 accessory belts to be on the safe side. It's
been almost 2 months now and I just started hearing a noise under the
dash up by the firewall when the A/C is turned on, It sounds like a
short burst of air lasting just under 1 second. Observing the noise
frequency, which is still sporadic on my 30 min. drive to work, the
sound now 1 week later last about 2 full seconds, still sporadic,
mixed with shorter burst as earlier. Can anyone tell me what it may be
and what to look for?


Ok, now the noise is immediately after switching the air conditioner
on after pulling out into the street. The sound is the same as last
time but just longer now, like 4-5 seconds and a lot more frequent
within the first 10 minutes, now that it last longer, when you first
hear it, it makes you think of a burst of air, but now it sounds like
water running thru a pipe like in a house. I'm not sure if it is a
pressure build up being released in a tube or a vacuum system that is
trying to seal off. I'm just not sure, but it is coming from under the
dash inside the car, not like a
squeaky belt under the hood.