Mighty mouse makes buzzing sound?

Just gone through the procedures of manually fixing the common problem of the mighty mouse tracking ball having stopped working due to finger junk/maintenance neglect.

As the Apple official recommendation of reversing mouse and working out any blockage didn't suffice, I found:
http://julianschrader.de/20071012-how-to-fix-the-mighty-mouse-scroll-ball-video/
...was the practical answer.

All good, so far. (for many it seems to be!)

However, having reconnected the scroll-wheel unit, screws, ribbon cables, clips, etc. and re-plugged back into the Mac, powered up, the mouse is now making crazy buzzing sounds and is automatically continuously scrolling upwards, unintentionally!

I have checked and re-checked the correct re-assembling of the four tiny magnetic stamps in the scroll wheel unit and all connections are established, but still the buzzing and scrolling. Also the 'ratchet' type of noise when scrolling is lost to quiet, but smooth scrolling.

Am really missing the luxury of efficient safari browsing and the zoom function, but as the mouse is 24 months old, I am wondering whether this is the reason and has partially bitten the dust.

Thanks in advance for any comments.