RX400H, 2006, about 75K miles.

I have noticed a lowish grumbling/droning noise and its been steadily getting worse over the last 6 months. The noise varies with the road speed, not the engine revs, so it could be tyres, wheel bearings, brakes or the output side of the transmission. Begins to get noticeable around 40mph and above, its really quite loud cruising on the motorway at 70. Tried driving with the windows down to see if it was the tyres, but no especially unusual noise coming in from outside - I am not sure yet but my best guess is that the transmission is getting noisy.

If the ATF fluid was low would a warning light come on for that?

A mechanic once said to me that just because something is noisy doesn't mean its broken. The car drives as normal, but its getting much louder in the cabin than it used to be. Anyone had a similar issue? Or ideas on how to narrow down the source of the noise?

 

Sounds like a possible Wheel Bearing issue.
Do you know how to check wheel bearings with a jack and a prybar and wiggling the wheel side-to-side and up-and-down?
Youtube has plenty of videos showing how. I recommend jacking up each wheel to check for abnormal movements/feels and sounds.

 

I'll jack it up and give it a wiggle, thanks.

Just checked a few youtube videos on wheel bearing vs tyre noise, its about the right kind of sound for what I'm hearing. Better a new tyre or wheel bearing than transmission repairs ($$$), fingers crossed.