k4 engine noise on over run

[all the gear]

Hello all ,my sons k4 has just started making a noise on over run .ie if I rev it to 3000 then shut the throttle the noise is apparent .Does anyone have any ideas as ,I thought possibly cam chain tensioner .
Any help would be appreciated

[DJ123]

what kind of noise is it . . . . . . rattly, metallic, knocking etc etc

[all the gear]

Hi ,It's more of a dull Brrrrr !