I get a rymthic hum at about 72-75 MPH?

[letstry1234]

I have noticed this after the break in period. (kept it slow for break in) I get a very rymthic hum at a little over 70 MPH. every 2 seconds or so..on then off. sounds like it is coming from the front somewhere. hasn't really bothered me a ton i have rebalanced the tires & rotated & also replaced the wheels, (not for this but to get away from the hubcaps). i can only hear it when the radio is off. very light sound. just wondering if anyone has the same sound. I now have 13,000 miles & it has never been to the dealer. i have done all my own maintance.,, 1,000 mi oil...3,000oil...5,000 syn oil... 10,000 syn oil. checked trans fluid properly.