Strange long squeal sound when I pedal hard - how to track this down?
This sounds like a bottom bracket bearing.

Check for looseness in bottom bracket by wriggling your pedals. Do they move in ways that they should not?

When the bearing housing fails, you both lose the glide of the bearing and also introduce bits of 'shrapnel' into the bottom bracket which rub and squeal with each crank, as the bracket deteriorates further, the noise and friction will increase. However, at some point, you'll grind all the shrapnel into shavings and will have scored the interior of the bottom bracket such that there will be more room and it will become easier to pedal and quieter, though it will still be far more difficult than a working set of bearings.
Bottom brackets are easy to inspect and replace. 
Good luck with the repair.