Engine not starting – rapid vibration noise from the starter.

Hi just wondered if I have a really serious problem that has developed over the winter and wondered if anyone can give some pointers.
At layup time .. everything all fine.
Just a standard 4 cylinder AQ171C volvo gasoline engine layed up according to the manual including fogging etc.
Come to start and hear a rapid vibration from the starter / solenoid but engine not turning over.
So far..
Replaced starter with brand new replacement.
Changed battery
Disconnected alternator and tilt pump.
Flywheel turns normally when turned over by crank nut on the front of the engine.
Same noise with spark plugs removed , flywheel turns over by hand as expected.
Propeller spins freely- i.e. not in gear.
I will check the relay circuit tomorrow but that looks ok at first test.
Just wondered if I have overlooked anything so any help would be appreciated.

[FONT=&quot]Hi - All sorted now..[/FONT]
[FONT=&quot]Turns out to be a very intermittent high resistance cable in the starter / relay circuit - devil of a job to trace only by replacing cables one by one.[/FONT]

Been debugging stuff all my life.... I learned the hard way... best NOT to start by replacing the most expensive part in the system first... just sayin'