Another possibility (if you have the 5HP24A transmission). The torque converter lock-up clutch is susceptible to wear once the transmission exceeds 100k miles. The LUC was designed without torsional vibration springs/dampers to save on installation length and instead relies on software control at low speeds to allow it to slip at around 3% to absorb torsional vibration. It only locks solid at higher engine speeds where TV isn’t an issue. The total LUC piston movement is only 1mm, so wear in the friction plate means the controlled-slip mode becomes harder for the control system to manage and the result is a vibration at lower engine speeds that sounds a bit like driving over a cattle grid. If you look closely at the rev. counter you can see the engine speed wavering slightly (usually only by a couple of hundred rpm) in sympathy with the noise.
Phil
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