under acceleration the engine will knock loudly as you accelerate harder.
the balancer is advancing the timing. look for an oil leak in the front seal
area as this deteriorates the rubber in the balancer. eventually this will
lead to the balancer coming apart as it did on my 95 gt

Big lar

I asked the question as I noticed a slight vibration while on a recent long
drive at speed. (you can feel it in the floor pan)  Very slight but
nevertheless there and it appears to be throttle sensitive. Trying to rule
out certain items of the many that could be the cause.

You can put matchmarks on the hub and pulley of the damper.  (BTW, on
the 5.0L it's a harmonic damper; the engine is internally balanced.)

If the rubber is deteriorating  you will probably notice the matchmarks
getting further apart as you put mileage on the car.

M dad's '96 GT developed a shake noticeable only on the highway too.
It turned out to be the driveshaft being out of balance.  $120 to
balance and get new U-joints and the car was smooooth after that!

BTW, I notice in your sig that you have under drive pulleys.  Most of
the UDPs I've seen eliminate the rubber-isolated harmonic damper.