Hi,
If it occurs around the same speed it might be a worn u/j or cv joint on the propshaft. Same thing happened to me (including going quieter with the clutch down) but with time it developed into a vibration as well.
Wheel bearings tend to get worse when taking a corner, eg turning left loads up the bearings on the right which rumble if worn or vice-versa.
Hope this helps.
Adam
TR7 V8 DHC Jaguar Solent Blue. 9.35cr Range Rover V8, Holley 390cfm, JWR Dual Port, 214 Cam, Lumention, Tubular Manifolds, S/S Single Pipe Exh, 3.08 Rear, 200lb Spax & PolyBushes all round, Anti- Dive, Strut-Top Roller Bearings, Capri Vented Discs & Calipers, Braided Hoses, 4 Speed Rear Cylinders, Uprated Master Cylinder & Servo, AT 5 Spokes and Cruise Lights, S/S Heater Pipes, Replacement Fuel Tank. No Door Stickers. Mine since July 1986, V8 from 1991