Yeah, that sounds a bit grindy!
I was wondering whether a pebble might have got caught between the pad and the disc, or whether a calliper might have seized, then the noise would probably only come from one wheel, and it might be persistent.
Heat and moisture can cause some rust growth on the surface of the pad, but this should grind off after you have driven for a little while. If the noise is persistent, I would probably get them checked. Maybe the callipers weren't properly adjusted when the pads were last replaced, and they have worn much faster than usual.
Do you notice anything odd about the discs themselves? Any scoring, rust buildup, etc.?
