Yes there is, not as good as BT but much better than most.
Here's link http://mu-2aopa.com

I would say close to normal speaking. Similar to my 340. Probably not as quiet as a 421, but not a dramatic difference either. I have not been in any airplane that is as quiet as my living room, so these kinds of questions tend to be a little subjective. My wife and kids prefer to wear headsets. Guy who owned it before me said his wife preferred to be in back and not wear a headset.

Minor technical niggle - in the direct drive engine, the compressor, prop, turbine (and everything else) are all one rotating group. The engine must idle at a minimum speed to keep the airflow adequate in the combustor section to keep the flame alive (and in the proper place) and the engine running. Prop speed is a side effect, not a primary cause. The PT6 can run just the compressor section at the rate adequate to sustain combustion without making the prop/turbine section run faster.

That said, the 4 blade props (on all 4 blade prop airplanes, PT6 too) can get into a phenomenon known as "reactionless vibration mode" where the blade pairs are vibrating at high magnitude but in exactly opposite direction so the net effect isn't noticed, and it can radically change the fatigue rate of the hub. So there was an AD which required the 4 blade MU2 airplanes to bump up the ground idle speed to help counter this (among other things like a hub redesign).

Are the 4 blade props still a 5 year OH item?

A per-airplane AMOC has been being approved lately that brings it to a 7 year item. If the FAA sees positive results from that, perhaps it will lengthen further.

If you want to nitpick (and I'll say why it's important), they were never a 5 year "overhaul" - it is a 5 year "inspection". The scope (and therefore cost) of the inspection disassembly is pretty similar to an overhaul, so most owners just said do the overhaul (no obvious reason not to). Most owner flown airplanes only fly maybe 100 hrs per year, so the overhaul was just due to the calendar requirement of the AD and not high hour props. However, it turns out one of the requirements of the magic word "overhaul" is that the blades be reshaped to the spec'd shape, which results in blades being ground each time. The blades are produced from the factory with about 3 overhaul cycles worth of excess material. So now, after roughly 15 years of the AD regime, a lot of owners are seeing their overhaul bill significantly "enhanced" by having to buy new blades (~$5K each x 4) since after grinding the blades no longer have enough material. My conclusion is that I will not do an "overhaul" until it is really needed, but will do just the required "inspection". Hopefully it should keep the blade bogeyman away for a while.

You explained it better though I would say that prop speed is the primary cause here, or maybe prop inertia. Take the blades off the prop and you could ground idle a -331 at a much lower RPM. The reason ground idle is so high is to provide the power to keep the prop from bogging down the engine.

Hmm. I will have to think about that a bit, but I'm not so sure. 65% RPM (for 3 bladers) is 65% RPM with or without the prop, and the airflow through the combustor (determined by the speed the compressor spins at) is at a particular velocity at that point, related to the speed of the flame front of the burning fuel. I'm sure one could maintain the 65% RPM with less fuel flow if there were no prop attached, but I'm not so sure that you would spin it significantly slower. It is possible the 65% could have been a little slower as a practical matter due to the speed the controller would have to react to prop load changes, but conceptually the engine needs to rotate at some minimum speed in order to support steady state combustion.

yes, over at cessna.org/forums you can learn a great deal about the MU2!

yes, over at cessna.org/forums you can learn a great deal about the MU2!

yes, over at cessna.org/forums you can learn a great deal about the MU2!

What's the ground idle like for the TFE-331? Basically same core attached to a (?) lighter rotating group.

Googling this a bit, not sure that's true.

Anyway ... are there any APU/ground versions of the 331 to compare operating limits? I'm guessing the 65% number would be more than trivially lower in an application designed, for whatever reason, without the need to drive a prop at idle.

Ground idle speed in the 331 really has nothing to do with the prop. In beta range the prop is controlled by the power lever through the prop pitch control which allows the pilot to manually set blade angle for ground ops. Ground idle blade angle is virtually flat. Engine speed is set by the speed lever and at ground idle is controlled by the under speed governer (variable 65-97%). The low side setting of 65% (varies somewhat by engine model) is a function of airflow required to sustain proper combustion. To the best of my knowledge prop mass or aerodynamic resistance does not come into play and required speed would vary little without the prop installed.

The noise issue is the first stage compressor more than anything.

John IV

What's the ground idle like for the TFE-331? Basically same core attached to a (?) lighter rotating group.

You had a very good explanation John, but I'd venture to say that while that first stage compressor is the cause of alot of noise on the 331 series, I'd guess the MU-2 noise has to be from the propeller. I flew the Metroliner/Merlin series with virtually the same engines (-10's on the 226's anyways) were not as loud as the MU-2, even with a minimum RPM of 71% in ground idle... It could have been the upside down engine directing the sound a different direction, but I have never heard a Metro whine like a MU-2..

Also, in Ground Idle mode, the propeller blade angle varies considerably, based on position of the power lever. The only AFM I have handy is for the slower turning -11 engine on the Metro III, but in Ground Idle, it varies from ~+7 to -13 degrees. It's only "virtually flat" when the crew is not commanding any thrust from the engine on the ground and starting...

Just my opinion,
Jason