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Update: Car had a tune up. Stuttering at constant highway speed disappeared.

The noise could be coming from the harmonic balancer on the crankshaft. Remove the belt and start the engine. If the noise gets better the balancer is likely to be what is causing the noise. Also, look at the front of the balancer and see if the you see cracks in the rubber as shown in the photo below. The balancer is made in two parts with the rubber separating them. When the rubber goes bad it allows the metal parts to bang together creating noise.