A similar noise happened to us...I think it was the Harmonic Balancer/bushing.
98GS - 3" DP & Cat back, 9" K&N CIA in FW, Plog, 3.5", 180* Canned tune. 14.65 at 96.9mph, w/2.37 60ft. on G-Timer @ 4500 elev. @ 51*.
87 Limited Turbo Regal- 12.04 @ 115 -Purchased New. Sold after 23 years and 115,000 miles