2015h: I've got a weird roof rattle that appears to be coming from the right-side of the sunroof. I've bought a new seal for the sunroof to see if that helps but the sound doesn't mimic the videos I've seen online so far.

Here's the weird thing: If I hold the passenger seat or the headrest, OR if someone sits in the passenger seat, the sound dissipates but does not entirely disappear. Still, the sound is coming from the roof or is resonating something in the roof. Any bump or rough surface agitates it.

These cars are known for a rattle in the sunroof. Mine had to be re-torqued at 35k but under warranty. It is where the sunroof frame mounts to the roof. A new seal is the best place to start but if that does not fix it you may be looking at a headliner drop and re-torque. When I did mine, the dealership was well aware of the issue.

Try lubricating the seal if you have not changed it yet to see if the noise changes.

Tilting roof would verify seal replacement if noise goes away. Working for Toyota roof pop creak over bumps was common had to retorque mounting nuts. It’s odd though you state can hold seat and noise is reduced are you possible changing the path of sound to your ear? Try moving seat all the way forward or tilt back rest all the way down what happens? There were plenty of squeak rattles you’d swear was from roof but noise was coming from dash and was bouncing off roof. Have you tried emptying all things from vehicle yet ie: loose change, sunglasses etc?

Probably not this in your case but another rattle source is a seatbelt that is retracted but twisted so that the buckle taps against the bottom of the "B" pillar instead of laying flat with the buckle towards the seat.

Is it the right front side?

Yes, the right front side.

Not an easy task to fix it. The headliner should come down and there's a kit from Toyota to fix that rattle. Not a very DIY job.

Also, putting window sealer around here will fix things:

While I’m still getting some creaking and rattling from the sunroof, I have significant less noise going over bad pavement after buckling the passenger seatbelt when nobody is in the passenger seat and by ratcheting the headrest to its most forward position on the passenger seat.

I still find the car dampening very poorly and that many of the creaks and rattles would be diminished if the suspension was doing a better job of absorbing imperfections. There’s rust on my strut bodies so a replacement is likely in the future.

Our 2020 ESh Lux makes a front passenger seat rattle periodically so I just lean over
and make a very small adjustment to either the recline or rear height and it goes away.