I have a Hummer H3 speedometer bouncing issue. Today, after driving on the highway at 70 mph for about two hours, the speedometer suddenly started bouncing and I heard a loud noise coming from outside. The RPMs are stable. When the car is stopped, the speedometer works fine, but after starting to drive, the same issue occurs again within 5 to 10 seconds. What could be the problem? I'm attaching a video.

 

Welcome.

YEAR and MILES???????

It could be your T Case speed sensor, or the internal Transmission speed sensor is going bad, its connection, wiring harness, corroded connector, even dirty cruddy fluid can occasionally cause inconsistent readings.

When was the last time you changed the Trans fluid & filter, and the T Case fluid?

Easy stuff first. Get under and check that the T Case speed sensor is properly screwed in all the way snug, check the wiring to it for rubbing, cracks or frays. Everybody with an old vehicle should have a can of CRC Electronic Cleaner. Use that on the T Case speed connector. Disconnect it, spray both sides, let dry and reconnect. See what you have then.

 

A bouncing or erratic speedometer is typically caused by a faulty Vehicle Speed Sensor (VSS) or damaged wiring resulting in poor data transmission to the gauge cluster. Common causes include a malfunctioning sensor or corroded connectors.