ok tick that off....pick the next cheapest on the list to look at

go 90 or so and try it in 2nd, 3rd and 4th and tell me if it hums in all of them. If it is only in 4th then you know it is a trans issue.

I only notice the humming in 4th gear, I have curiously dropped it back to 3rd quite a few times to try and replicate the sound but can't hear it and don't get the vibration...

Now you mention the trany I do remember a while back not long after I got the car, I got the auto serviced and I fondly remember dropping the car off to the automatics place, I went for a walk and came back and the mechanic that did the service for me walked up to me shaking his head and said "she's a dying one" (refering to the auto) and showed me the blacked trans fluid and showed me the filter that came out of the box and it had a small amount of metal shards in the filter... He pretty much made out as if the transmission was totally rooted

I was quite concerned at first, but I had my dad with me and when the mechanic walked off to do something else my dad explained to me that when he had his EL falcon (the opt20 that was handed down to me) He took it in for an auto service at a different place (years before) and the fluid that came out of the box on the opt20 was also black and the filter had metal shavings in it. He was told essentially the same things as I was and told the auto was on it's way out. The opt20 had around 110,000km on it when my dad was told this and he never had a problem with it (except a small amount of shuddering if he was driving it hard) We sold the opt20 a couple of years back with 267,000km on the clock and the people we sold it to are still driving it with well over 300,000km on the clock and they tow a horse float with it... it's still the original box and they haven't had any issues.

Likewise I haven't really had any issues with the auto in my current EL falcon however like the opt20 it does shudder somewhat if you really put your foot down...

Sorry for the long post but I just thought I would share this incase it may be related to the humming and vibration issue I am currently having. (and no the opt20 didn't have the humming or vibration but we were told the same thing about the auto's)