Ok, we just bought a new HVAC system (11 grand worth). Heats and cools fine. But, the compressor/fan unit outside makes loud thunks and hisses then sounds like gravel in a cement mixer. We have had the techs out 3 times and of course it doesnt make any noise while they are here. They say its the unit going into defrost mode. But it kicks in and out every 30 seconds or so. My wife hears is very late at night when its cold.

Any ideas? The unit is 6 mo old with a 1 year warranty.

I'd try to video the symptoms for them.
I'll assume they have verified that the refrigerant charge is correct and other variables are within factory specs.
You then can share the noise with a couple more other service techs for their opinion. You have 6 months.

^^^That sounds like a good idea. Don't know if I would have thought of it. Would also work for a noise your car was making that, of course, would not happen when the mechanic was there. Or any other piece of machinery. I like it.

Good idea! Get some evidence.

Tell the techs you don't care about the warranty and to just get it fixed. You might get a different response.

Make, model and serial... ?

They are providing you with a 1 yr labor warranty.
The equipment will have a standard 5 yr parts warranty, with some extending it out to 10 yrs.

The thunk/thud and hissing are normal for a HP, when switching into defrost mode.
While they are there checking it, are they forcing the unit into defrost mode (to replicate the noise)?

Gravel in a mixer? Not normal.

What's kicking in and out every 30 seconds?
The unit starting and stopping, compressor kicking on/off, hissing, thunks?
^^^ The 'every 30 seconds' statement, makes me think the unit is low on refrigerant due to a leak (not good).

Is it a scroll compressor?

If so, it can make a sound unfamiliar to you, as it is not pistons pumping in cylinders.

Concur that frequency of sounds suggest an issue.