(So good links, but old to the) TIP of TIGHTEN THE MOUNTING BOLTS IN THE RETURN GRILL AREA can help vibration, but also consider a toothbrush CLEANING (GENTLY, DRY FIRST, THEN w/ spray cleaner, so that you do NOT bend the blades) of the FAN BLADE/ SQUIRREL CAGE BLADES of the blower and / or the condenser fan, can help REDUCE VIBRATION. Good TARGET FOR SPRING CLEANING if not a current bad issue.
2000 Conquest LE 6266, Class-C on 99-E450SD, 6.8, 2v, V-10
I replaced my AC unit with the Atwood Air Command. Best investment in our sanity ever! Runs SO much quieter ... on LOW you can barely hear it. Love the digital thermostat and as an added bonus ... draws less than 10 amps!