knocking sound
 

Hi there, I need some help. I recently serviced my Subaru legacy2000 model and was having issues with my boost, was unable to boost over .5bar, so I cleaned all my pipes n solinodes and the boost returned and was able to get .9bar. However after I had parked for a while and went on the road I noticed a slight knocking sound and as my rpm's increased so to did the noise. I haven't got any engine check light on dash and when I have it at idel its silent.
Has anyone had this issue before and could anyone advise on possible cause n solution?