This may be the hanger bearing in the middle of the driveshaft. If it appears more at cooler temperatures, varies with the speed of the wheels, is only present when the car is moving, and the hanger bearing has never been changed, then it is likely the culprit.

You have some options:

Most expensive: Replace the bearing (worse still, have it replaced)
Cheapest: lubricate the bearing
Unwise: do nothing

To lubricate, safely jack the car and put one end on stands (or raise it on a hoist). Use a hypodermic fitting on a grease gun and inject some grease into the bearing by putting the needle in under the outer edge of the dust cover between the inner and outer race of the bearing. No disassembly of any kind is required. Results are immediate and can well last several years (then repeat the process). Total time required, including jacking, finding tools, clean-up etc.: about 15 minutes. Cost: nil
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'61 122s B16, '67 122s B18, '76 245 B21F, '81 245 B23E, '90 745 B234, '95 960 B6304 (not counting parts cars)