'86 Aerostar XLT, standard length, all options, 183,000 mi, pristine condition. I need helping finding the source of a vibration that appears when driving at about 2000 RPM, peaks about 2500 RPM, almost disappears above 3000 RPM. Since 2500 is 70MPH, it is quite bothersome on the Interstate, and besides, something could be falling apart! The vibration is heard more than felt, and manifests itself as low frequency cyclic chortling sound that appears to be from the driver's side front. It reminds me of the sound you hear from a bad bearing when you use a screwdriver or stethoscope to search for it. I can't tell if it's happening or not in the driveway, as the other noises mask it. However, for the first millisecond at the instant the engine is started, it makes the same deep guttural sound. It happens at 2, 3 & 4 gears at the above RPMs and their relative road speeds, so I expect it is engine speed related and not suspension, rear end, etc. I've eliminated the following, as they have been replaced for one reason or another since the noise/vibration began: rebuilt trans with new torque converter, at which time the flywheel bolts were SUPPOSEDLY checked, new power steering pump, and no change in vibration when the van was driven with the fan & clutch removed, vibration damper looks good --- but how does one really check it? Engine and trans mounts look good when stressed, although they are my next target if I can't find something else that makes more sense. Also, when on the Interstate and I go over a quick rise or undulation in the road, at the instant I go up the rise and while I am at the top, the noise is less. It resumes during the downward motion but does not get worse unless I am on rough road. All of these symptoms cause me to think I've got the engine resting on the frame, but the motor mounts look fine. Can anyone help?