NAD "noise"

My NAD 3120 amp, when I switch it on, makes the following sounds:

1. Immediately there is a singled "Doof" across both speakers.
2. About a second later there are 2 soft "whoop" sounds from each speaker, one always before the other.

So,

2. I am used to, and have always heard it on most of my NAD amps. No worries.
1. sounds worrying - it is too loud, and pushes out the driver cones to their fullest extent. Is this just a spark suppressor cap needed across the mains at the on/off switch, or something else?

Any advice much appreciated.

Thanks,
Johan