Resonance!!

[Mckorkins]

A peculuar thing is that at around 55-60mph, there's a drumming sound that Daisy makes. It's only between these speeds it's noticable. I liken it to a German bomber with engines out of sync: woo woo woo sort of noise!!, I've checked all the obvious things like the UJ's (now replaced unnecessarily) and had the wheels balanced etc etc. The engine and gearbox mounts are as good as new and there's no excessive play anywhere in the axle or gearbox end spline. I can only assume harmonics come into play for some reason. I don't think it is a balance problem. However, It occurred to me that an electric fan conversion kit fitted last season could be a possible cause. I just wonder whether the free windmilling fan could be upsetting the harmonics somehow. Any thoughts?

[Mckorkins]

And I replaced the exhaust system for good measure in case the baffles were loose in the silencer. In the event, there was no need!

[Bill_qaz]

Wheel bearing?
Cardboard over front grill to eliminate the fan theory .

[Mckorkins]

Wheel bearings all sound and recently replaced. Cardboard solution not advisable in summer!!

[Mckorkins]

Actually, Bill, on second thoughts it's worth checking all wheel bearings because they could fail even though they were replaced a few 1000 miles ago. Sorry to appear being a smart a**e in my answer just now.

[Bill_qaz]

Moggy is well over cooled cardboard for test run will do no harm. Takes a bit for the stat to open. If worried partial cover just to disturb air flow to test your theory. Or temp cable tie electric fan blades and disconnect power, no fan required while moving.

[Mckorkins]

Good idea. Will try.

[kevin s]

what you describe sounds like hetrodyning, it happens when you have two frequency generators with similar frequencies, the combination of the two gives a new much lower frequency sounds as the peaks and troughs coincide.

As you describe its what you hear on multiple engine piston engined aircraft.

I would suggest the most likely cause is something on the wheel ends, you've ruled out wheel bearings, what about tyres?

[Mckorkins]

Actually Kevin, you are right, thank you. I think the problem was the rear wheel bearings all the way along. The diff was rebuilt some 20 years ago, but possibly the bearings weren't renewed, or were of inferior quality. I have got some "German" made bearings as replacements in stock, so I will fit those.

Bill, I tried blanking off the electric fan with cardboard and removed the fuse to disable it before I found it was a bearing problem after all. So much for my wild theory about syncro problems and harmonics! Often it is the obvious and nothing more. We'll see what happens when the new bearings are fitted
Thanks chaps!

[Bill_qaz]

Number 1 on my first reply, glad you found it and it's a straight forward solution.

[Mckorkins]