- please don't use all caps. It is very hard to read.

Where were you driving? Did it persist? Can you repeat it all the time, on different road conditions? Does one of your mufflers have a hole?

A low rumble I have not heard. There is often a high-pitch whine that is characteristic if the IMA, we think.
If it sounds very bass-y you are not hearing 1600Hz directly- more like a subharmonic or a low engine resonance that is getting excited there.

If I had to guess, based on no other information, the active noise suppression is not functioning properly at that engine speed, and sending the 'wrong' noise through your speakers (even with radio off it is active). That is asusming it wasn't road noise or some other external wierdness.

Please let us know what the service dept says!