Rain Noise w/ Standing Seam over 2x4 sleepers ?

I am getting quotes for installation of a standing seam roof on my A-frame cabin in the Catskills area of NY. The existing roof is plywood sheathing with asphalt shingles and in good condition. There is a bump in the roof which seems to be from a previous addition to the home. I've talked to several metal roofing contractors and the plan developed was to install 2x4 sleepers or purlins across the roof, over top of the existing shingles, and the sleepers could be feathered to eliminate that bump and provide a perfectly flat new roof install.
Then the metal roofing would either be installed directly onto the sleepers, or another layer of plywood installed, then metal roof onto the new plywood. If I install the metal roof over the sleepers, it will be spanning between sleepers with an airspace behind the metal roof panel to the shingles. I am wondering if I will be likely to have excessive noise issues with this install due to metal being spanned across between sleepers with 1.5" airspace behind, and not damped with any material behind it. The benefit to omitting the plywood would be less $.
Thanks in advance for any help. I've read plenty of anecdotes about metal roof rain noise, but haven't found any examples similar to my specific question with the existing roof remaining and the airspace behind the metal panels.

,Adam

Excessive noise, not likely. Noisier than if over solid decking with the decking tight to the metal...yes.

The presences of the plywood layer and shingles will certainly dampen any noise whereas you will get a bit of a steel drum effect with an open design.