There has recently been quite a few weekend closures on that section so its possible that the track has been relayed.
Thanks Southern Stag. On the journey from Reading to Reigate I realised there may be another (simpler!) explanation.
The units I travel on from Thatcham to Reading are (from memory) 2 unit class 166s. if the front area to the left of the first set of doors (behind the driver) is std class I always sit in there. My understanding is that the engine for the unit is effectively half way between the two bogies. So if first class
is at the front I often sit above the engine which is bound to be nosier...so this could be at least part of the reason.