I'm on a new Hybrid with a 1 x 10 setup. Mid range components Shimano derailleur etc. What I have noticed is that if I pedal hard in a mid to high gear and the step off the gas to freewheel the chain goes slack suddenly before it catches up. It's almost like the freewheel doesn't engage quite quick enough or the rear derailleur isn't tense enough to take up the slack and you get this kind of chain slap.

What is this most likely to be or it it just normal on this type of setup. I have never noticed it on any of my previous bikes.

Thanks

Not normal. You mentioned the bike is new so the first thing you should do is take it to the shop you bought it from and let them inspect it. Anything you do to correct it and fails, the shop can blame you for the problem and not honor any warranty leaving you with the cost of repairs. Having said all that, if not under warranty it sounds like a case of lack of, or sticky lube in the pawls of the cassette hub so it doesn't always engage but could be something else too.

First step would be the shop as mentioned.

If you want to look it over just to get a feel for it then you could unmount the rear wheel and check the freehub for drag and for proper freewheel pawl operation. It's possible that it's a little draggy for a few different reasons. If you spin the cogset and freehub smartly it should spin for quite a few turns. If it doesn't spin at least a half dozen then something might be rubbing or it has too thick a grease like lube in it. But that's something the shop should be checking before you start tinkering on a new bike. It's OK for you to pull the wheel off and give it a once over. But stop before you reach for any tools for the sake of the warranty.

Just to make term use more standard a freewheel/freehub/one way clutch "catches" when applying power and the drive "detaches" when not pedaling (coasting).

So I would describe the issue by saying that the ratchet doesn't disengage quickly, if I understand the problem correctly. This is the same issue I have with one bike when in the smaller rear cogs. The chain's leverage acting on the ratchet is less so if there's any "stiffness" in the ratchet/freewheel/freehub that tries to continue to drive the cogs. Top of chain stay chain slap is a usual result. The cause is some binding/friction/spot gabbiness of the cog carrier on it's core. Could be as simple as a spoke guard rubbing, a tight seal or as bad as poor manufacturing in the unit.

I agree that if the bike is new and bought from a LBS you should take it back and have them look at it. If the bike was sourced from a long distance/on line "dealer" than you will likely find you are on your own in doing the solutions. Either way, contacting the source is the first step. Andy

You could invest in a bike stand, a few basic tools and start trying to replicate your condition. This is the Bicycle mechanic section of the forums, everyone saying take to the bike shop obviously just wants to be safe with suggestions. I'd say educate yourself on what you have and don't be intimidated by extremely simple mechanical mechanism.
Once you go down the rabbit trail and figure out what is the contributing factors you will be forever connected to your bike, you will be one with it. Don't rely on some hourly employee to have any real concern for something they will probably consider normal for a 1x10. were talking about a 11-46 cassette and a medium cage derailleur, their is going to be some chain wiggle depending on what cog your in. good luck never give up.