Rumbling noise.... possible auto gearbox issue???

[Whiskeyonesix]

Hi Guys,

Got a problem... I was going to post this in the driveline section, but I thought this might be a general AU issue...

When I'm driving, and I coast ( in gear ) I hear a rumbling noise from the underside of the car. Noise goes away if I accelerate or brake. No vibration at all.
Only noticable when driving (60 kph+), not if I'm just crawling through traffic.

I thought It might be the exhaust, but it's not a "knock" or anything like that, or maybe CV joint - but theres no noise when I turn the wheels so fairly unlikely..

I'm thinking maybe diff or gearbox? Car is upto 213,000 on its original gearbox, so it's probably about due to go...
I had the brake discs and pads changed all round about 3 weeks ago - noise started just after that.... any connection do you think? Mechanic who did them checked the car over, serviced it, and gave it a clean bill of health - nothing wrong with it back then...

I'm off work for the next couple of weeks so I'm going to start trolling around a couple of garages to see what they think - I just wondered i you guys could give me an idea first....