hi guys i have a astra 17 cdti that as a slight knocking noise not diesel

you can hear it when you,ve put your foot down appears after you ease off or put clutch down

doesnt do it all the time and if you sit at lights or a junction you can hear like a whearing noise

checked cam caps for wear nothing could it be a hydraulic tappets or do these engines have them roller tappets

timing belt just been changed last week and thicker oil put in as it was doing this before

any help

cheers guys thanks for any advice