I'm assuming since you only said shift into 3rd, then highway driving that your car is an automatic. I'd probably say it's a bad torque converter. When you're at freeway speeds the torque converter locks up and spins as a unit, when under load the impeller and turbine inside the torque converter spin at different speeds. I'd probably guess there is a clearance problem inside the converter causing there to be grinding, quite possibly the turbine and impeller are rubbing against each other. This is of course, my guess. Keep us posted as to what they find.
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2005 Suzuki Hayabusa GSX1300-R
1980 Ford Thunderbird - 255 V8
ported heads, 5.0L ported stock headers, O.R. H-pipe and Flowmaster 2-chambers, dual roller timing chain
hi-po Mack Truck hood emblem 
1985 Mustang GT 5.0L T5, F-303, GT40p, headers, off-road h, flowmasters, MSD stuff, etc.
Sold 02/06/04 
1989 Mustang GT ET: 13.304@102.29 mph (5-24-03)
Sold - 1998 Mustang Cobra coupe, 1/4 mile - street tires: 13.843@103.41 (bone stock)
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