I have a '98 328i and I've noticed it's pretty loud with the A/C on... During normal operation, it's quiet, but as soon as I press the A/C button, I hear it kicking in, and this hissing noise starts coming from under the hood. It's not a high-pitched hiss or anything, just really loud and continuous... kssssssssssssssst........... As soon as I disable the A/C, it's gone.

It seems to be coming from the intake side of the engine, but I can't tell exactly where. I think it's coming from the front half of the engine bay; my friend seems to think it's coming from the rear half.

Also, my A/C clutch seems to kick back on after only 8-15 seconds of being disengaged each cycle. I've had the car for almost a year now, and I use the A/C once in awhile, but not very often; I'd assume if I had a leak in the system, it would've run itself dry by now, right?

I don't think that it's the compressor because that's on the passenger's side, and the sound's coming from the driver's side. And FWIW, the A/C works like a champ.

Any help or suggestions would be greatly appreciated...

i have this same problem =x

check ac charge

check ac charge
:lol :lol Man its like your the second person I have seen giving out info with out any clue what so ever .
O.K let me see if I can help .
First of its not the freon.
As far as your explaination , it sound to me , that it might be your tensioner , or the tensioner pulley. Does it happen when the car is cold ?
When you turn the A.C on you are actually placing a load on the engine so the tensioner is actually has more strain and pressure on it .
Do this turn the ac on and then go and spray WD40 on the pulleys and the tensioner and see if the noise goes away . If it does , then you need to replace the pulley and the tensioner . Im sure thats what it is .
By the way the pulley and the tens is right by your alternator pulley, on the right hand of the front of the engine . You need to take of the alternator coolin tube to get a better shot at it. Good luck.

A "short cycle" of the A/C compressor, 8-15 seconds, could mean a low refrigerant charge.
When the compressor turns on and off with a short internal it is because the suction side pressure drops rapidly to the point where the low pressure cutoff switch indicates that the pressure on the suction or intake side of the compressor is too low.
Low pressure is a good thing on the suction side but too low is not good since it could cause an icing condition of the evaporator core.
There is also the possiblity of a blocked or partially blocked AC system which could be caused by a number of out of the ordinary occurances within the system. To cover these occurances would be more of a topic for an auto refrigeration site.
Of course do not rule out the very real fact that the idler pulleys on the belt drive could be going. Also the clutch hub bearing on the AC compressor can wear or start to fail which allows the AC clutch hub to start to run out of true. This will cause the AC clutch plate to scrub on the clutch face. This will definitely cause the "hissing" type of noise you describe.
I have an 01 Chevy Express van in the shop right now that is having the AC compressor changed because the AC clutch hub bearing is very worn. The hissing noise is very loud and of course the noise goes away when you engage the AC because the AC clutch is engaged and not scrubbing.
You may have also fractured one of your AC compressor clutch plate springs. The AC clutch will still work but since the AC clutch plate is not longer running square to the face of the clutch hub it will make the scrubbing noise. Seen it.

andrew

:dunno Could one of his hoses have a hole in it and the freon is being "liberated"? That sounds like a logical solution to me. If this is the case, it would explain why the compressor is doing short cycles, becuase its losing all of its juice! Maybe this is the cause, correct me if i'm wrong! :dunno

:dunno Could one of his hoses have a hole in it and the freon is being "liberated"? That sounds like a logical solution to me. If this is the case, it would explain why the compressor is doing short cycles, becuase its losing all of its juice! Maybe this is the cause, correct me if i'm wrong! :dunno

I've already considered this, but ruled it out quickly seeing that the A/C still works and has been making this noise for as long as I've owned the car (about a year). If I'd had a leak, I'm sure the system would have needed recharging to keep working.

It's not a typical "hiss," more of a deep "kssssss..." Hard to explain, but it really sounds more like bearing resistance or metal on metal but not too harsh. It really doesn't sound like a high-pressure system leak.

My whole point of this post was to see if there was some sort of theme to my problem. like if many others with similar BMW's had this issue. I know it is not something that can be pinpointed with the information provided. Just looking for some guidance.

Maybe I could shoot a small video with my digi cam and post it; maybe that would help?????