49er

Have a hybrid triple 8 speed Tiagra rear derailuer new installed, bike less then 1 year old.

Once excelerating to a constant speed or cadence i get on my downstroke a Thud or hammer noise, as if the chain is loose and catches the cassette teeth, just like when you about to start pedalling.
This is the best that i can describe the problem.
This hesitation or hickup or thump is very annoying and is present when i'm at a set cadence when pedalling. Another analogy is if you skipped a gear but happens at every down stroke when pedalling the cranks. Its like trying to open a combination lock you pull and pull but it won't open.
This problem came just after the bike was taken to the LBS. When you turn the bike over and turn the cranks it all looks good and smooth but it's a different story when riding on the street.

I'm i imagining this or is it a problem that can be fixed like a loose chain? How would a chain get loose in the first place.