Modern Vespa

OK, unfortunately, I'm not sure how good you can hear the actual sound, but it starts at about 3.5 second in and last for a few seconds:

https://vocaroo.com/i/s1cwWYMx4J2K

There are 3 main noises here: the main engine noise, the wind noise, (which are both normal) and this sound I try to describe here, the one I have a question about:

It's almost like if you briefly held a plastic drinking straw against a fan for a second. Like a slight buzzing sound. This will sometimes occur when ever-so-slightly holding the throttle, usually at about 18-19 MPH, but sometimes a bit faster, maybe 25-30 MPH.

It seems to come from right under my feet, or actually slightly towards the back and the right, but underneath. So I would assume something with the muffler, but not sure.

I have tried removing the metal plate on the outside of the stock muffler, like maybe it was shaking, but that seems tight and ok. I also don't think it is the license plate cover at all. I have also loosened and re-tightened the muffler clamp.

I think my next step is to remove the muffler altogether and check for something loose.

Am I just being overly sensitive to the noise? Maybe. I'm mostly curious what it might be.

Anyone have any ideas?

The castle nut cover and split pin which hold on the back wheel can sometimes rattle.