I got a new wheel set. Vuelta Corsa SL. Changed 10spd shimano cassette from RS10 wheels to the new wheel set. I left old spacer. And placed the new one as well. Now while pedaling it's all fine. Stop pedaling and start again and I get this "clang" as if something became lose and is re-seating or something. Doesn't sound like chain or anything. Pretty convinced something is up with the hub/cassette interface. Any suggestions? Thanks in advance.

Remove the cassette lock ring and ensure that the cassette overhangs the freehub body by a couple of millimeters, so that the lock ring clamps the cassette without bottoming out on the freehub. If it does not overhang then additional shimming is needed. Then grease the lock ring threads and torque to the recommended value , typically 22-36lbf-ft or 30-49N-m, which is a surprisingly-large (to me) torque.

To reinforce this point we see loose lock rings frequently. Just last Sunday a fellow rider (at The Ride for Greg in Milton, ON) suffered this fate. he had to call for the support van and luckily was able to get the lock ring tightened (after reorienting the small cog which had also rotated around the freehub body so the splines didn't index). Andy.

Lube the new freehub

What the OP describes sounds exactly like the loose lockring I once had.
That was how I learned to always use a torque wrench on it.

Awesome. Thanks for the tips. I'll try and fix it asap.

If you check the cassette and the lockring is properly tightened on an overhanging outer sprocket (or spacer), then consider the possibility that nothing is wrong.

When you coast the wheel overruns the freewheel. Then when you start pedaling, there's a short dwell until the freewheel catches back up and engages at the first stop. Various designs, have more or less dwell (space between clicks) and some designs can be noisier when engaging. So it's entirely possible that you're simply hearing that hubs normal sound of reengagement.

I was thinking the same thing

Okay. I rechecked everything. Lubed. Torqued properly. Removed the 1mm spacer leaving behind the 1.85 and the tolerances looked better. Just like old setup. And everything seems fine. But this random and startling clang still occurs. It's not normal because I can't recreate it easily. The engagement of the hub is silent compare to this noise. I notice it's more apparent with more force being applied to the pedal. But it only happens after I coast then start applying force again. But not every time. I'd not know where to begin but I've only noticed since I changed to the new wheel set. Caad9 optimo 1. dura ace and ultegra mix. I'll take it to LBS tomorrow because I can't stand it. It's almost like something is moving 1mm with enough force to resonate through the bike. It happens in any combination of gearing. Rear axle is tight. Nothing else has changed other than wheelset.

Just a followup to my issue. It's the wheel spokes. Maybe burs between the spokes where they run together or where they are laced through the hub. Applied some tri flow to those spots and seems to be waaaaay better.