Hi
When we turn on our heating the radiators up stairs sound like they are struggling to fill and it sounds like there is no water in them it sounds like they filling up but after about five minutes the noise goes is this normal as this has only just started happening.

FHW (forced hot water) or steam?
Baseboard or radiators?
Usually it's air and it needs to be bled out. Air can build up very slowly just from what's in the water, or if it happens fast, it might be a tiny leak letting water out and air in.