I recently bought a second hand 54 reg Scenic and when I used the key card to lock the car it emitted a high pitched noise/sound/whistle. While looking for the source of the sound I leant on the bonnet. I found that every time I put pressure on a certain point on the bonnet the noise stopped, but when I took my hand off it started again. I guessed it must be some kind of contact breaker, so opened the bonnet and took a look. I spotted two contacts on the underside of the bonnet and guessed they were being held too far from the main body, on the edge on the opening I found two round rubber adjustable dampers (presumably to stop the bonnet banging when closed) I screwed them both in slightly and it cured the problem.
It's worth checking this before taking the car to the garage. I guess the high pitched noise is to warn you that something isn't closing properly.

Replacingalternatorbelt

Replacing the ECU is not a difficult job, just ensure that you wire it correctly, and always unplug battery cables before replacing or unplugging the ECU.
If the scend hand ECU is from same model it will be compatible, not all meganes have the same module, ask to retailer for information on specific models.