A little advice pleas

So the truck in my sig the other day starting to have a tick come out of the drivers side on top of the engine. it almost sounds like an air leak but i cant find anything leaking anywhere. so i hooked my smarty up to it and there was a dtc that said cooling fan speed, so i cleared it and it has not come back yet. my mileage has dropped a little bit but not much only like 1 mpg or so. I just changed my fass filter and went with a 2 micron and it almost sounded like the tick got a little bit quieter, so im thinking fuel related. took it to several diesels guys and they all say" never heard that noise out of a cummins but i would have your injectors checked?" So im gonna pull em tomorrow evening. there is a shop round here that will test em for 30 a pc if i pull em. what do yall think? My truck got 132000 an as far as i know they are stock injectors. any help is appreciated.