The reason for this thought is why did the shaker motor start of its own accord about 5 secs after powering up - i assume this is NOT part of the self diagnostics? 

It could well be - I know that on Dirty Harry the self diagnostics runs the motor for the gun on start-up.

I would try turning the pin on with the motor disconnected next time - but definitely take a look at that shaker motor as a possible problem area.