Chainsaw

Paddy buys a chainsaw, two weeks later he takes it back to the shop and tells the shop assistant, "when I bought this, you said it would cut down 50 trees an hour!, I've only managed two trees an hour!"

The assistant says "let me see" and starts it up.

BRRRRRRRRR!!

Paddy jumps back in surprise and says

Spoiler:

"what's that noise?"