Need an explanation on expansion valve and A/C operation (96 Accord)

Hi again, all! I need some advice understanding and perhaps fixing a bad A/C
system in my '96 Accord. The A/C has worked fairly well up until a few days
ago. Starting LAST summer, we noticed a kind of weird intermittent noise
coming from inside the car under the glove box when the A/C was on. Very
hard describing noises but this sounded something like a sizzle. I don't
want to sound 'leading', but it reminds me of the kind of noise you hear on
the nozzle of a spray bottle for like brake parts cleaner. Not the noise you
get while spraying, but the noise you hear after you stop spraying when the
propellant kind of builds up on the nozzle as you spray and it then
'evaporates'. Anyhow, this noise starts several minutes after starting the
A/C, lasts for 10 seconds then stops and comes back a few minutes later. If
you shut the A/C off, the noise ALWAYS occurs (again, for about ten seconds)
but only once after the A/C is shut off. If you turn the A/C back on while
it makes this noise, it immediately stops, but will return after a few
minutes.
Now, I did have a Honda mechanic look at it briefly while carrying out a
different repair and he stated it is the expansion valve.
This was a couple of weeks ago and I wasn't in any hurry to have it fixed as
the A/C seemed to work fine - it just made that noise. The only other
unusual observation was that it very often blew water vapor through the
vents, which I not only thought was normal, it's also a little cool (no pun
intended). I now feel this visible exhaust seems so excessive, it is not
normal as it does it even on dryer days.
Today, I hopped in the car; it was sunny and hot. The A/C was very
ineffective while driving in the city. After I got on the highway, the A/C
seemed to blow cooler air and was sufficient to keep me cool but only on
full blast. It had also turned overcast.
The last time I checked, I did see water pooling under the car (I didn't
check it today though) . I have had a clogged drain tube in the past which I
did notice just by hearing the water sloshing around and I managed to unclog
it myself. So I don't think that has anything to do with my problem now.
So what I want to know is, (as I believe my mechanic's diagnosis) :
WHAT IS AN EXPANSION VALVE? What does it do and can I fix it myself? I have
fixed clutches and brakes on my cars before so its not like I'm useless with
tools or anything. But, is this valve in line with the pressure, i.e. does
the coolant has to be captured first with special equipment?
I have done some major searching already in this and other N.G.s and haven't
really seen a concrete answer to this question, only on how to diagnose if
it is an expansion valve that is faulty. However, I have seen some 'fixes'
that involve cleaning and removal or bending of sensors etc. and wondered if
there was a inexpensive fix for this. Finally, is it likely that I have lost
coolant through this faulty valve? Like I stated, this noise occurred all
last summer and to date this summer but A/c seemed to function. Today it was
noticeably inefficient. I did see a post that stated just because I have a
bad valve and no cool air doesn't mean I have lost coolant. Just need to
know what I am in for dollar wise.

Thanks to all that took the time to read!

P.S., Does anyone know where or if there is a coolant sight port on this
car?

 

You might want to learn to use search engines for this kind of inquiry.

http://www.familycar.com/Classroom/ac1.htm#THERMAL%20EXPANSION%20VALVE

 

Thanks for that info. Yes, I can see how it could be called a hissing noise.
So, did you replace the valve or simply recharge the system? Let me know if
you get a leak.