OK - here's the gen on the low frequency drone.
I am testing a Secdem screen at the moment and get exactly the same thing in certain screen positions.
The disturbed air coming off the screen is hitting the side of your helmet. Your earplugs are also touching the inside of your helmet. As a result, there is a direct transfer of the noise into your ears.
I am running with fitted earplugs with speakers in and have the same problem of noise conductivity via the plugs into my ears with the Secdem screen in more upright positions - in particular the low frequency drone/reverberation you mention.
To experiment, I put the Secdem screen in the most drone-inducing position, and fitted snot-green cheapo earplugs (you know the ones). Much much quieter as they weren't conduting any noise into my ears. Back on with the moulded ones and the drone is back - and it is v.v.noisy.
Try this - put your helmet on without earplugs, and tap the side of it next to your ear. Then repeat with your moulded earplugs in - if the sound is louder and more "inside" your ear it confirms the plugs are touching the helmet inner and conducting sound into your ear.
Most likely your earplugs are too proud of your ears - maybe whoever made them can machine some material off the outside without affecting the inner moulding. Alternatively, look at trimming some of the "comfort" padding next to where your ears go inside the helmet (obviously dont trim the harder white foam).
Cheers
Andy