Scan for transmission codes and pressure-test the transmission while it is shifting hard. Drain the fluid through a rag to check for metal, and service the fluid and filter. A restricted filter or weak modulator valve is possible, but hard shifts on this transmission are often caused by a failing pressure control solenoid; replacing it requires draining and refilling the fluid again. Do not ignore the problem, as continued hard shifting can cause internal transmission damage.
The owner of a 2000 Bonneville with 49,000 miles reported occasional hard shifts and a different whining noise, though the transmission usually shifts normally. One reply suggested routine fluid and filter service and checking the modulator valve. Another said hard shifts are usually caused by a bad pressure control solenoid, recommended checking for computer codes and pressure-testing while the problem occurs, and advised draining the fluid through a rag to look for metal. Fluid service is due near 50,000 miles but may not correct the hard shift unless the filter is restricted.
