inline fan noise

ok so i just replaced my booster fan with an inline i got a pretty good deal i think. its a demo so the guy gave it to me for 60 bucks its a 4". but he said it makes a noise every couple seconds like a ....errr..errr.errr like somethings is lose perhaps?

anyone have any ideas... thanks!

p.s

fan works great...

mount it to a stable surface