Another possibility besides the motor mounts is the generator fan. I kept hearing what sounded like the fan hitting the radiator while driving but I couldn't find anywhere on the blade or radiator that showed markings from hitting. I finally got the noise to reproduce while under the hood and it was coming from the generator fan. The nut had stripped which was allowing the fan to wobble just enough to make periodic contact with the housing. Good thing I caught it before the pully/fan came clear off.

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