I've had 2 Previas now that developed a whistling sound while accelerating. On both it turned out to be the muffler. The 1st one I purchased brand new from Toyota (was expensive). The 2nd time I bought a used muffler from the pick-n-pull yard. It looked well used, but it was cheap (like $20). It was original Toyota (stainless) & I've been running that one for 6 or 7 years and no sign of a whistle yet.