Vocalization While Grooming?

My 20 pound, neutered ginger tom has done this most of his adult life.

When he grooms, he often makes a noise that we refer to as "oinking", it is a sound something like a pocupine makes.

I have never heard another cat do this.

It is not like he is in pain, it just seems to be his routine.

He is 20 pound by nature of overall size, he isn't really fat; I have wondered if this is something some larger cats do.

Lol, Hemmy does the same thing. I call it grunting. Hemmy just seems to enjoy it! Hemmy has a stout frame but at 14 lbs isn't a big cat. He does have to do a bit more work to reach say his back since he isn't as long as our other kitties.