HVAC hissing at the evaporator

I have two identical 4 ton units, 1 upstairs and 1 downstairs. The downstairs unit runs like a champ, however, the upstairs unit hisses at the evaporator. The house is new (3 years old) and have had the AC company that installed it take a look at it several times. It is an intermittent problem, and seems like every time they come out it runs quiet. In March the compressor died, and was replaced. I still have the problem, however, the hissing has become louder and the compressor will shut off violently (I hear a thud as if the pressure was way too high.) The only other clue to the problem is that it hisses all the time but tends to get louder in the afternoon and peaks in terms of "loudness" around dusk (if it occurs.)
I can see myself having to replace the compressor again if the problem is not rectified. Any ideas what this could be? I need some advice on how to direct these repair guys, or at least be armed with some knowledge when I shop around for a new AC repair company.
Thanks,
Will
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If "YOU" have to be the one to tell a HVAC company how to repair your unit then you have called the wrong company. You should only have to describe the problem and any related info. Then its up to a competent company to do the repairs. Id be concerned with refrigerant charge, refrigerant metering, proper installation practices and of course, proper ductwork, equipment and air flow. There you have it. Now get busy. Bubba
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Will wrote:

Will;
Quite frankly - you need 6 vital signs for a competent air conditioning tech to tell what's "what" with your unit. Get those readings and head on back to alt.home.repair and I'm fairly certain someone will be happy to answer your needs on this issue.
--
Zyp



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you didnt mention age of this particualr unit, but it sounds like it has a tev
that may be the hissing you hear . and that may be directly related to your problems , but - we can't fix it from here , we can't troubleshoot it, hell,we can't even see it as someone has noted , if "you need to direct" your repair company then you should be looking for another , we can't help you
try alt.home.repair
BYE !

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You don't need an HVAC company..... You need an exterminator.
There's a fucking snake in thar!
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