Do you know what reading you had for the A/C pressure? Also, trying raising the RPMs while parked and check to see if the compressor shuts off. The noise you hear may be what is listed in this TSB
A/C - Module Emits a `Tick-Tock' Noise
File In Section: 01 - HVAC
Bulletin No.: 01-01-38-007
Date: May, 2001
Tick-Tock Noise From HVAC Module Assembly (Install Spring)
1997-1999 Buick Park Avenue, Ultra with RPO CJ2
2000-2001 Buick LeSabre with RPO C67
2000-2001 Pontiac Bonneville with RPO C61
Some customers may comment that the Heater and Air Conditioning (HVAC) Module Assembly emits a repeating "tick-tock" noise.
The "tick-tock" noise may be due to the two air mix (temperature) actuators on the Park Avenue HVAC module and the mode and air mix (temperature) actuators on the Bonneville and LeSabre HVAC module oscillating/cycling when they have reached a position on the HVAC module.
Install a deflector spring, P/N 25732764, in the covers of the two air mix (temperature) actuators on the Park Avenue HVAC module and in the covers of the mode and air mix (temperature) actuators on the Bonneville and LeSabre HVAC module.
Black 2000 SSEi, Silverstar Headlights, Angel Eyes fogs, 3rd brake light overlay, hi-flo cat, drilled 180 degree thermostat, Intense FWI, PCM, shift kit, and 3.4 Pulley. Solid front mount.