It seams to happen most as 1st gear shifts into 2nd (20mph-25mph) and then again as it shifts into 3rd (35mph-40mph)
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2004 Hyundai Santa Fe Question: rattling, squeaking as automatic gears shift
Answer #1DaveJHM May 08, 2011, 01:14Master
For the record - 20-25 mph is way too late for a 1-2 shift. You are probably feeling a 2-3 shift from 20-25 and from 35-40 a 3-4 shift...or you may be feeling the torque converter engage.
In any case, with the noise - it's pretty impossible to do anything from the Internet on this one -- you'll need to have a good tech hear the noise first hand and give you advice.
Click that link for some shops near you!