[700rpm]

It could be pitted gears. Open it up and take a look at all you can see. If you have pitted gears you’re fine for a long time (I drove my Tudor for 34 years with a whining 2nd gear). If the gear teeth are badly worn or sharp, you won’t get a lot of life out of them, but it depends on how much and how hard you drive it. I wouldn’t suspect bearings because your noise is only in 2nd. Eventually you’ll have to tear it down. It’s your call now or later.

Oh, and by the way, some of the new gears, even the well made USA gears, will whine. They aren’t as well matched as the originals were. My coupe whined for the first 2000 miles with new Mark gears, but it’s getting quieter.