This may sound pretty out there, but it's an idea. Assuming that you're driving an automatic, in a modern/tiptronic transmission, the next gear will start spooling up which would make for a slightly increased noise. However, this is nothing that should really be noticeable. I am also assuming that if you're at 3500rpms then you're working it a bit above an average take off therefore making the trans hold on to the next gear slightly longer...
Like I said...it's pretty out there.