2002 3.0 X5 engine stops while driving

[kfm]

X5 E53, 2002
Engine: M54
Trans: GM 5L40E
76K miles

The car has seen little driving in that last couple of years, just occasional runs to stores etc. never more than 30 miles each trip.

Yesterday I took a longer trip. 60 Miles highway driving 75% at ~70 mph, 25% with traffic jam going as slow as 25 mph. Arrived at destination no issue.
Two hours later, start driving back. Driving slowly (<40mph) for about half a mile, the car lost drive. I gave it some more gas, but rpm was not responding. Then the engine just stopped and "service engine" light came on. Coasted to a stop. Attempted to start the engine twice. It started to run, but then immediately dies. No error msg other than the "service engine light". Had the car toweled back home.

Next day I tried to use a Foxwell scanner to read diagnostic data. But for the three key items
• Motor Electronics2 DME
• Transmission Control EGS
• Transmission control unit EGS
I got no information because the scanner says no communication with these units. Tried multiple time with key in different position.

Finally I decided to try start the engine again and it started right away. But it was running rough especially when the engine speed start to drop from the initial higher rpm. Eventually it settled to a smooth idle. I tried to engage both reverse and forward gears, could move the car ok, but didn't want to risk to drive it more than a few feet. During this time, the "service engine" light didn't come on.

Tried to use the scanner again with engine running, still no communication. All this took about 15 mins, then the engine started to run rough, and the "service engine" light came on the engine shut off itself.

One other interesting thing was that when I turn the key to run position, I hear a high pitched whistle, last may be 10 seconds. This was something I never heard before, it is on top of the normal sound a X5 makes when you turn the key to run position.

Now I have no clue what is wrong with the car, hope someone could give me some hint.

Thanks!