The return of Toyota would be great! It's been one of my favourit marques in sportscars. Take a look at their CanAm prototypes or the very fast 88C-V group C car and the TS010 or the GT-ONE
I don't think though,that it will be a superquiet machine. That is,as long as they don't use a silenced dieselengine...
And a diesel is probably not the best option for a hybrid for diesels are heavy and a hybrid system too,so a combination of the two is unlikely,I think.
As for the noise,takin' away the beautiful sounds that different racingcars produce is castrating the sport imho.
9 times out 10 it is not the envirement around the track that is being harmed but a bunch of complainers with nothing else to do,that try to take the fight to tracks and racefans...