Odd Vibrations

[Snoggledog]

Before we go any further, no batteries are involved.

I have a vibration coming from the NS front wheel at 40, 60 and 80 mph (indicated) but can't seem to work out what's causing it. Oddly the vibration and associated harmonic hum does not appear below 30, from 42 - 58 mph and from 65 - 80 mph. The vibration can be felt through the passenger footwell and through the steering as well. There's little to no vibration when going round a left hand bend when the wheel is unloaded and a lot more vibration on a right hand bend when the wheel is under load. So far I've checked:

The NS front wheel to see if all the nuts are fully tightened. Check. No problems there.
The NS front wheel to see if there's any play (for wheel bearings). No problem there.
Tyre pressures. All ok.
The car is not pulling either way.

So at this point I'm stumped. I don't think it's wheel balancing as the issue has been getting progressively worse over the last 3 or 4 months. I don't have anything more than a wheel jack so can't really lift the car enough to get underneath and have a good look.

Any thoughts?

[garrykiller]

get the wheels balanced?

[MX-5 Lazza]

As above. Sounds like a wheel balance problem to me.