Another AC problem

About a month ago, Cathy told me her AC was making noises. I checked and sure enough, the noises were coming from the compressor. We still had AC when moving along. The sound just got louder so I knew it wouldn't go away. Last weekend, I changed the compressor, filter/drier and expansion valve. With the system disassembled, I back flushed the entire system. Including the compressor, there was very little oil in the system and no sign of debris. I did notice that the clamp/retainer on the refrigerant behind the expansion device was upside down in relation to the book diagram, so I assumed the problem may have been caused by someone working on the system and never adjusting the oil properly. I added the proper amount of oil according to the manual. I left the vacuum pump on the system for several hours, then added a static charge of refrigerant into the system and let it sit overnight, since it held a vacuum so well, I assumed it would hold pressure, which it did. Sunday afternoon, I reclaimed the refrigerant and drew the system back into a vacuum and let it sit about two hours while I did some cleaning on the car. Vacuum held, so I charged 700 grams (plus about 20 more-the manual specifies 700+50tolerance) using a digital refrigerant scale. System ran very well, very cold, very quiet. Then it started making a very intermittent rumbling noise that lasts about 5 - 10 seconds, maybe about every five miles, especially when idling at a stop. Just a low frequency roar, not very loud and not like a mechanical knock. The compressor clutch stays engaged and the cold air is not interrupted. I have noticed several related threads, but hope we can all get to the cause on these. I am lucky in that I was a marine engineer for most of my professional career and have all the nice goodies to service AC systems. My best guess as of right now is some resonance in the expansion device, but am not sure. I will follow any leads and get back after trying everything this coming weekend. Thanks. Cathy starts school this week and needs her mechanic back.:)

Are you sure the noise is coming from the a/c compressor. Vw's do have problems with there cats and rattling.

I'm fairly certain it is not the AC compressor--that is brand new and the noise was never mechanical--just a resonant rumbling. The noise has become less frequent and annoying. Since my helper removed the belt without marking direction, it was most likely installed in a different rotation or flipped inside out. Might be belt chatter. I will probably change the belt just for grins. I also have a clunk/knock on the driver's side when shifting from drive to reverse. Not loud, just annoying, too.

Can the evap drain valve chatter?

I have no other signs of cat failure, but will definitely check that out, too. I did little car work this weekend. Too hot here in Virginia. I'll keep checking any advice and let you all know what I find.

Thanks

I would check the fans when they come on, It may be the bearings in your high speed fan causing the rumbling, especially when idling. The only time your high speed fan comes on is at idle(usually), and newer VW's have lots of problems with noisey fans.

Thanks a million! I hadn't even considered that. First time the noise happened was first time everything got good and warmed up and we slowed to a crawl doing a three point turn around. Then primarily when sitting at a light, very rarely at speed. Occasionally a very brief (1-3 second) low volume rumble, seemingly without any related cause. It has been terribly hot here, the AC blows very cold, only problem is the rumble--I will give the fans a good check. Thanks again.

Back again--the problem is all but gone (low frequency rumble in AC) but is still a bother when it does show up. It has happened after the car is turned of following heavy A/C use. This leads me to some sort of chatter in the expansion device when equalizing pressures. The clunk/rattle noise was the transmission side motor mount cracked up under battery cable tray/wiring run. Easy enough to change.
Any assistance will be appreciated. No other noise from any other component and the AC is ice cold. Just that rumble--I may stick some cloths/sponge between the expansion device and fire wall to see how noise changes. Next step appears to be change out expansion device. Thanks again--oh-the fans seem to be just fine.

I may have an opportunity to check this problem further this weekend--I'll post whatever I find out. This vehicle is getting more use now that it has become the family backup. I couldn't pass up the deal I was offered on my daughter's dream car 07 NB yellow w/gray interior and all the usual goodies plus, but I lost my '99 Jetta diesel (145000 miles) in the trade. I'm happy, but now awaiting the diesel return next year.
On the AC problem, a friend with a Toyota Tacoma has the same rumble when he uses his AC on high. I'm looking forward to this fix.

Time to put this one to bed once and for all--finally. I was in the process of selling the '99 in question when the excessively hot temps brought the rumble back. I followed Hako's sound advice and hooked up the gauges again. This time in the proper weather conditions. Only when the temps are around 100 Fahrenheit (32+Celsius) did the discharge pressure of the compressor hit the high pressure cutout, the clutch disengaged, then repeated. I recovered refrigerant enough to keep the pressures in the normal rangs and the problem was solved. I would recommend in hotter climates, the charge be kept at or on the lower side of the 700 +or- 50 gram range. I weighed in 700+20. Not good. Thanks to all who chimed in.