Since the balancer is not running true, I would pull it off and check for cracks alongside the keyway.  That is the typical failure mode.  The snout gets cracked and they start to wobble, ultimately destroying the timing cover seal and the key itself in the crank.  Replacing that key requires timing chain removal.  So, I would play it safe and inspect it from the inside out.  If you don't find anything, and you have a correct 69 balancer (unique to that year engine) I would replace it anyway.

If you pull the pulley off and take a picture it might help.  If it is original, the rubber can also go bad on the outer ring/mass so you have two potential failure mechanisms of the damper to consider.  Pull the damper and inspect the key and crank and do the runout check from just beyond the edge of the keyway to the opposite side just before the keyway.  There may be (but shouldn't be) a ridge/lip along the keyway.  You should be awfully close to zero at the crank but there may be some movement as you turn the crank due to bearing clearance.  Another check would be to set up the dial indicator at 12:00 and then carefully lift/pry the crank with a wooden block to get a sense of the bearing clearance to ensure that your runout is not due to incorrect clearances or premature bearing wear.

The drone or potential "beat" you are describing can happen when you have two closely spaced vibration sources - you airplane analogy is a good one.  In a car, this is often due to the engine, driveshaft, or even a drive wheel speed/frequency matching up with a natural frequency of some other lightly damped system since we usually don't have two engines like an airplane.  My previous Mustang setup had a natural frequency of the exhaust center section that would be excited at 49 MPH.  Just above or below, a beat would develop and right at 49 it would shake pretty heavily.  A beat occurs when two vibration signals become nearly in phase with each other and add together and then a short time later they become out of phase and subract from one another.  If you measure the number of beats per minute (a watch is a good tool for this) you will learn the difference between the frequency source and the resonant component.  The driveshaft I had taken out of the V8 donor car was a mess so I had a new shaft made and it pretty well cured the problem.

A beat that is due to interaction with a natural frequency can be cured by either moving the natural frequency by stiffening or weakening the system (there are more elaborate methods but take a little more engineering).  Often weakening can be more effective in a car because the vibration forces are lower at lower speeds (they decrease at the rate of the square of the change in RPM).  So, if your exhaust system is very stiff or rigid it may actually be contributing to the problem.  Alternatively, if you correct/minimize the vibration source, the beat will also improve or disappear.  In addition to your damper, you can also pull your driveshaft and have it balanced.  A way to see which is at fault is to get into your drone condition, note the speed and then downshift to a lower gear while maintaining the same speed.  If the drone goes away, it is engine speed related and your balance issue is in the engine. If it stays, look to your driveshaft or a wheel.

Other problem areas that can cause problems like this are motor mounts and the transmission mount.  If you look under some of the newer mustangs you may see some weird masses that are hung off of the transmission or the rear end.  These are often tuned absorbers that are designed to minimize problems at certain driveline or engine speeds that are identified during testing.

TMI, I guess, but solving a vibration problem can be tricky without measurements that can identify the exact frequencies in play.

Had that happen on my three bolt pulley.  The damper was secure.  The pulley had one missing bolt, one broken bolt and one lose bolt.  All three holes were elongated.  I was able to get the broken bolt out of the damper, then I rotated the pulley by 1/6th and redrilled the holes.  I put the new bolts in with blue locktite and I was off and running.

The pulley had been making noise like a bucket of bolts but I could never find it.  I had taken it to the muffler shop to have a dual exhaust installed.  When they fired it up it was making a horrible noise.  We opened the hood and the damper pulley was actually rubbing on the water pump pulley above it and throwing a shower of sparks.  I was just moments away from leaving the shop and getting the car on the fwy.  That pulley would have let loose and created horrible damage.

I had the Mustang towed home and fixed it all in the garage.

Last edited by boomyal (9/14/2013 4:32 PM)

Yes, I should have said the balancers tend to crack alongside the keyway.  And, like ultratang, I have learned the lesson the hard way about replacing the woodruf key when you replace a cracked balancer.  That is how I know you must remove the timing chain to do so.  I bought two new balancers, because the rounded woodruf key cracked the first balancer almost immediately.

The 69 specific crank pulley and balancer match each other in the way they fit together.  The 69 pulley has a lip on it fits into the front of the pulley,a nd the 69 balancer has a machined groove or inset for it to fit into.  All other years have a raised ring on the balancer that fits inside the big hole of the pulley.

It is late right now so I hope I have that correct, but it is possible I could have it reversed.  It has been many years sionce I owned an original balancer OR pulley.