As Mark suggest, the butyl damping pads are excellent for attaching on panels, much better than felt. They will help attenuate the noise and reduce the frequencies. Felt is remarkably poor as a sound reduction material.
From experience working with noise, vibration and harshness (NVH) engineers on various vehicles, I would suggest using butyl pads to reduce noise from panels and structure borne noise, basically the road noise and vibrations transferred around the body panels and structure.
The are many products available. Dynamat is one of the better known brand names. Henkel sound deadening products are widely used by the vehcle manufacturers.
Search the web for Dynamat. The specs are available from this link.
https://www.dynamat.com/wp-content/uplo ... et_Web.pdf
I like the ingenuity with thinking about using imitation lead flashing. It could be ok, and it's cheap and readily available so worth experimenting with. It's likely to be quite a bit less absorbent than the purpose designed sounds dampening pads and you would need more of it to get the same performance.
It's also worth considering a sound absorber material - typically this would something like a open cell foam such as Armacell RD240 (the Rolls Royce of sound absorption foams) with a heavy barrier material such as a 2mm rubber sheet attached to the outer surface. This is a great combination to reduce airborne noise levels from the exhaust, transmission, tyres and more. Every open hole on a car allows airborne noise in. The noise finds it's way past all the grommets etc and the sound absorber material can significantly reduce it.
As Peter mentions, foams can by absorbent, some such as the Armacell are good at resisting water ingress, whereas others can be way too absorbent to consider. Info on Armacell RD240 is available here:
https://local.armacell.com/fileadmin/cm ... KROI-1.pdf
I did a quick search on the interweb but didn't find any particular guides that could be helpful.
What I did find was this article in Miniworld that seems useful to show where to put some of the material.
https://www.nkgroup.co.uk/wp-content/up ... rticle.pdf
I have no afiliation with these guys and they seem to have a good product range, and possibly have a good understanding of the subject:
https://www.carinsulation.co.uk/
Rattles tend to be more related to specific parts vibrating due to the engine vibration, road inputs or other sources. For example, as the vehicle travels down a road, the vehicle movement may trigger engine movement that feels like a vibration, or it can trigger vibration in the doors, seats, other interior parts and much more. This vibration relates to the mass, (installed) stiffness, damping and location of the parts. To eliminate the rattle change one or more of those factors...
Hope this helps.
By the way I have no links with any of the products mentioned above!
Mark
