Hello Harold and welcome to the forum.

I agree with Phil's recommendation regarding the alternator. However, if the alternator is the culprit, it does not care if the vehicle is in motion. So, you can do all checking while the vehicle is in the driveway.

If you cannot duplicate the noise in the driveway, or affect its characteristics when adding additional electrical load, then you have effectively ruled it out.

Please attempt to find the source of the noise with the vehicle stationary. If this is not possible, because the noise has vanished, report back and we will attempt think it through again.