sounds very familiar, mine used to do it at very low revs by holding the throttle in a fixed position and when going into a slight incline. if it stops almost dead on 56 mph its most likely TC as thats when the lock up clutch kicks in i think, also if the rev counter waves up and down slightly in time with the rumble, that's another sign, as hippo says, much lower mileage than is common, i think 100K + usually
mine used to change up when you might expect it to change down up a hill with light revs and it used to rumble then as well but not always in top gear
i had my torque converter rebuilt at sussex autos. has been great for many thousands of miles
Torque Converter Suppliers | Torque Converter Remanufacturers | Automatic Transmission Parts Distributors and Torque Converter Remanufacturers - Sussex Auto Parts
although these people are probably best all round
Welcome to the website of JP (Automatic Transmissions) Ltd in Somerset, UK
RRphil or Bemble is god when it comes to these, i dont use the term loosely either, he could literally strip one and rebuild blindfolded
Saint is next in line i would say,
(smoke and arse spring to mind)
have a read through my thread below if you want to scare yourself
http://www.landyzone.co.uk/lz/f10/oil-leak-gearbox-229433.html
bottom line is if you get somebody to do it i would guess £1k - £2.5k depending on whether they convince you to rebuild your transmission as well
you could search Dr tranny shudder fix if you are bored
as saint says, an oil and filter change might help especially if any has leaked out dropping oil pressure
good luck
