98 Golf - noise at 35 rpm

Hi! This is my first post here, although I've learned a lot lurking. I am the new owner of a 98 Golf GL and I noticed that when it's at or above 35 rpm it makes a loud humming noise, like it's working too hard or something. I didn't notice it before because the exhaust was so loud but now that the exhaust is fixed I can hear it. I've noticed it in both 4th and 5th gears, 4th starting around 45 mph and 5th starting around 80 mph. Is this normal? Engine problem? Muffler loose? I just had the 120K service done a few days ago, as well as having the exhaust work done. I really can't pinpoint where the noise is coming from either...

Any help would be very appreciated, TIA!

Is it engine speed or vehicle speed related?

Is it engine speed or vehicle speed related?

I'm pretty sure it's engine speed because it happens at different vehicle speeds but always at the same rpm.

Could it be a loose exhaust? I hope it's not an engine problem!!

Does it seem to loose power at higher RPM's, like a bad/clogged catalytic converted? Or, just the noise.