Something is wrong with my engine, please help.

So yesterday I was driving my '69 beetle home. The ride was about 40 minuted on a 100+ degree day. I make this drive daily without issue, but today was hotter than usual. I got home and when I went to drive again 30 minutes later the engine was making an odd noise. When the rpm went medium to high, the engine would make this loud humming sound, and the car vibrates. It only happened at high rpm and got worse the higher it went. The car sounded fine if I shifted a bit early. I tried again when the car was cooled and it still makes the sound. It does it in gear and out of gear, with and without the clutch. I think it is part of the engine that must be spinning poorly. any ideas? I got a free weekend and don't mind going in for a look. Thanks for the help.

Update: Well, I solved the issue and found out that I am stupid. Last week I was installing engine deck lid risers and didn't bother to re-attach the wires to the license plate light as it doesn't work. Instead I just tucked them behind the engine. As you may have guessed, the wires got sucked into the fan and ripped them off. This length of wire caused all of the issues. As soon as I felt them and pulled them out, the engine ran fine again. Sadly, this was after about 3 hours of troubleshooting. Thanks for the help everyone!