Hum/vibration from rear

[Keegzzz]

98 vt auto.. So it started happening every now and then when I was going up a hill and going around 70-80 and the car wanted to shift up to 4th but instead just made this humming sound from under me towards the rear of car (gearbox?) and wouldn't want to shift and if I took my foot off and put it down slightly heavier it'd shift up and be fine but it's occurring quite often now and even on flats and a very slightly vibration is occumpaning the hum and has started doing it between 2nd and 3rd. Is it obviously the gearbox or something else? It shifts fine most of the time.. Feel free to ask questions for more info, thanks in advance

 

[kais3r]

Does it make this hum in park when you slowly rev the engine up and hold it at the same rpm as when it should change gears and doesn't?

It could be a gearbox prob but not all of the symptoms may be related to that, could also be engine or exhaust vibration, might even be an exhaust blockage.
Could be any of a number of possibilities, but if it's only happening in specific gears then it's likely the gearbox, so try it in every gear with same acceleration and other conditions, hills etc and try to hone it down to something.

 

[kyled2012]

I have a very similar issue in my VT, was told by my local Holden dealer that "there's no way it's an issue with the auto"...
Since then have had Engine mounts, exhaust brackets and a few misc parts replaced but still have this strange hesitation and noise.

I'd honestly almost given up on figuring this out before you posted, I hope you get to the bottom of it!

 

[Zom]

Have you checked the transmission oil level?
Symptoms like this could be caused by a low level.

 

[hotrod]

maybe if you meditate while it does it hum.............hum............hum................. it might go away,
like when was it last serviced , condition of oil is it burnt ,blocked filter , fluid level .

slight vibration is that your tail shaft centre bearing , maybe your extension house bearing , go and get a check up.

 

[hotrod]

if the engine is in good order and not misfiring, the trans will be at fault.
the lock up clutch for o/d is slipping causing the shudder, by manually putting back to 3rd,will not allow o/d to engage thus avoiding slip syndrome.
no amount of servicing will cure it, the trans will need to come out and be repaired.
common problem with 4L60E trans.....sorry for bad news

can be your coil pack to spark plug leads & plugs.

I replaced everything on mine when it was playing up on the magic 80km hr.
things I replaced, trans mount , uni joints, coil pack , plugs , it turned out to be the leads breaking down under load.

 

[Keegzzz]

Recently (within last 3 months) I've changed spark plugs, leads, coil packs, oil filter and engine oil but ill have a look at the trans because I have noticed a slight oil leak so perhaps the oil is low... I think I'll get a trans service done and see how that goes, will post findings