Mysterious Rumble at 65mph!

[mmenhenitt]

Hi, have a 1969 2000 Mk 1 Automatic which is great in all respects except that at 65mph and above a mysterious rumble kicks in that vibrates up through the two front seats. The body is rigid and there is no vibration through it. We have fitted a new propshaft and new inner and outer bearings on both rear half shafts. All hardy spicer joints also renewed. All courtesy of Chris Witor, who is also baffled!The diff is quiet at all speeds and the auto box is smooth and changes gears without problem. There are no shakes or vibrations up through the steering column/wheel.
Am at a loss as to what to try next to get rid of this rumble. Any ideas?
Mike Menhenitt.

[CAR]

Hi, did you check the gearbox mounting when sorting the propshaft and other UJs?
Try lifting the rear of the gearbox up by putting a bar between the bottom of the casing and the gearbox crossmember.
Also is the exhaust clear where it goes through the nearside trailling arm subframe, any touching here will send vibes through the car.

Colin

[mmenhenitt]

Hi Colin, yes all the baove have been checked thank you, and all ok.
Baffled!
Mike.

[englishbull]

Flexi plate?
May have an hair line crack in it as I have had this problem. I had the flexi plate go and fitted a NOS one and all appeared fine but I got the vibrations which as Colin mentioned I thought was gearbox mount or exhaust but all was fine. So fearing worst the engine was stripped but all fine and so decided to check the flexi plate and that was when the hair line crack was discovered. Then fitted a reinforced one off Chris and problem solved.

[owen1183]

Are the anti vibration straps in good condition and correctly configured?

Are they the original type, or later type (beefed up)

How about tyre shape, are they old tyres which may have gone egg shaped? (although this would be apparent at all speeds)

Have you tried changing direction when the rumble occurs, to see if it might be the wheel bearings (even though you have replaced them, they may not be adjusted correctly? Also I but new rear wheel bearings in which still rumble... bad quality)

Is it exhaust resonance, are all mounts in good condition?

Quill shaft bearing?

is your prop shaft operating in the correct alignment? I.e. do you have the early type diff, with lower nose piece? has your suspension sagged, altering prop shaft angle?

[Addiston]

I had a problem with my then automatic 2500S a number of years ago which only manifested itself at certain throttle openings. It was traced to the exhaust touching the gearbox cross member as the engine rotated a little under load. At low throttle it didn't come into contact. At heavy throttle it came into firm contact, but wasn't noticeable (no noise). At medium throtttle, however, the exhaust vibrated off the cross member and made quite a racket

The exhaust bracket on the bottom of the gearbox was bent back to the right shape (up a bit, as it had sagged) and all was well. The front engine mountings were also later found to be a bit sloppy, which wasn't helping restrain the engine's desire to rotate a bit.

It might not be relevant, but worth looking at.

Regards

[mmenhenitt]

Thanks to you all for the replies. Certainly, much of what has been suggested has already been looked at and found to be fine. The one area that hasn't is the Flexi Plate and possible hair line crack. Probably because I am not 100% certain where or what this is! Not come across this before.
The rumble is not like a constant hum it is a noise like a deep rumble/vibration of something going round and round and only comes up through the front seat at 65mph and above. Up until then it is smoothness personified!!
keep your suggestions coming, all very gratefully received.

[englishbull]

Flexi plate is in the autobox and when mine went vibrated and sounded like exhaust vibrating.sounded like metal on metal

[tony]

Had a similar thing in my TC some years ago. Quite a bad vibration at certain revs.Thinks, must check drive shaft universals in the weekend.
It wouldn't wait that long,went over a railway crossing and bang extension housing flange broke where it was joined to tube.,,,Hope this isn't your problem.
Tony.