Check if a critter built a nest in the air box or exhaust. Had a mouse pack the muffler full of dog food on a car one time and it sounded like that. PPPPPPFFFFFFFFF.

Also had a tech leave a shop rag in the air cleaner housing on a diesel truck. Same thing. Would run but not rev. Tried to leave the shop and told them it wouldn't go. I don't think they believed me, till they found the rag in there.