Mercedes Benz C Class when starting i get a clicking, nocking noise for 3 seconds then goes quite. but they noise is always there

Where is the noise coming from , engine , inside , dash area etc. ?

Definitely coming from the engine. Sometimes it really load and sometimes its not to load. Noise always goes but doesn't do it all the time

If you haven't had the engine oil and filter changed for a while , worth trying that using good quality oil and filter as oil pressure may be a bit slower reaching top of engine .

Had a oil change and filter at local garage and put in some treatment to clean engine and that didn't work.

A lot of people , including myself do not like or use those engine
" cleaning " products as it can loosen of muck which is then sent round the lubrication system and can end up anywhere .. Some recommend another oil / filter change again after it has been run a while .

Thanks for your help but do you think it could be a lifter causing the issue

Dependant on the noise , a sticking lifter is quite common on all cars with hydraulic lifters . Possibly lifter cleaner your garage added ?