On a grand every individual damper when lifted and of course when all dampers are lifted together by operating the sustain pedal, make an annoying 'hishing' noise.

It is certainly the often bespoke whooshing noise caused by the dampere felts touching the strings when moving upwards [i.e. flat dampers produce the same hishing].

What would be the remedy to overcome this issue?
? Easing the damper rail bushings; bending the damper wires?

I do not intend to do anything myself. Just to be informed as whenever a tech suggests doing something I am sure he is on the right track.

Many thanks,

schwammerl.

You and your piano should move to Madison, WI where all the RPTs are above averge. (Just a joke...)

The sound can be minimized, not eliminated.
First identify the offenders.
This noise is usually more associated with the wedge felts when too much felt extends into and beyond the strings when the dampers are at rest - this can be cured by trimming the excess.
It can get worse when the humidity increases.
Check strings for rust.

Gene,

Whooshing by the wedged dampers would appear to be the most logical explanation but I think it cannot be.

As I said in my openening post you get that typical sizzzling rubbing noise also on the NON wedged FLAT dampers.
It suffice even to raise these one by one by hand to get it.

No the strings show no rust; the grand it fairly new [9 months].

schwammerl.

I did not miss your words about the flats - just ignored them as it is a bit unusual.
The flats either share a wedge on one side and the wedge needs attention or they are deeply string grooved or they are traveling when they lift. Whoosh from damper felt is just the felt fibers brushing the wire. Sometimes ironing can help fibers lay down flat.
If the flat whoosh equals the sound made by a wedge then the problem with the flat should be obvious.
One other possibility is that the damper wire is touching the string but this would not be a whoosh noise.
Guide rail bushings should not make a whoosh noise. Bending or turning the damper wires would be done if the dampers are traveling when they lift - damper regulation.
It is easy enough to replace a flat with a new piece of felt to see if the noise stops.
Maybe your tech will bring some new felt when he comes to diagnose the noise.

You might try isolating the dampers from the damper tray. Do this by lifting the dampers with the damper pedal and then engaging the sostenuto. Then you can operate the damper pedal and associated trapwork and tray independently of the dampers. If the sound is still there, it is in the pedal, trapwork or damper tray. If absent, it's got to be the damper felts or the guide rail. I would have a hard time believing it to be the guide rail.