Hi folks, i've been dealing with this issue since i picked up my 02 4.6is a couple years ago. With the air conditioning running, over 3,000 RPM, i get a loud vibration from the right side of the dash. it starts vibrating at exactly 3k RPM, and stops just under that speed. It's a loud buzzing noise, not something you can really feel. It sounds almost as if it's in the dash or just beyond the firewall. If i keep the RPMs in that range for long enough, sometimes it will stop. P/O believed it was the compressor. I'm tending to agree but figured i'd post it up in case anyone else has had this issue before. I'm just wondering if it could be an unsecured AC line that vibrates behind the dash, or maybe the expansion valve. Perhaps the RPM dependency is related to the variable displacement of the compressor, and something changes above 3k RPM?
P.S. -worth mentioning, I replaced the condenser this year due to a leak and recharged the system - this made no change to the vibration.