We recently took a 2017 suburban in for 100k service which included bearing, brakes, fluid, wire harness, and new spark plugs.

immediately after the car has a whining/humming sound somewhere in the 30-40 mph range that goes away with continued acceleration.

the shop that did the service said it’s nothing they did and to take it to a Chevy dealership because it’s the transmission. However, there is no jerk or noise between gears. No power loss, etc

appreciate any advice

 

Sounds like the shop won't budge or accept any possibility that it's their fault. The local radio show ASE master mechanic would say this only can be pinpointed as to cause after that cause is found and fixed. So after the dealership or second independent shop fixes the issue, THEN you can determine if it is related to what the first shop did, and then confront them for adjustment/compensation or seek small claims court.

Sometimes, stuff just happens at about the same time. About a decade ago I set off for a 100 mile trip in my 1998 Frontier, found the charging light on, and the alternator was bad. Took off alternator, took to the parts store, walked out with new alternator. While there, water pump started leaking due to no tension on the WP seal due to belt being off the alternator. Back to store, picked up water pump. Got home, started water pump R&R, noticed fan clutch bad, so back to same store for fan clutch. Guys behind the counter were laughing, knew I'd cleaned up and changed clothes before each trip to the store. Later, I asked "my" ASE mechanic I use once a year about such scenarios, and he said it does happen, and especially tough if it's a new customer who might think he was merely trying to run up the bill !!!

 

I do most of my own repairs, but the mechanic I use in a pinch has a set of microphones he can attach under the vehicle to find out what is making a noise. They seem to work really well!