Does the be quiet! Dark Rock 6 CPU Air Cooler support liquid metal thermal grease?

Yes, it is compatible with liquid metal thermal grease.

What is the noise level of the be quiet! Dark Rock 6 CPU Air Cooler at maximum fan speed?

The noise level is only 31.1dB(A) at maximum fan speed.

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Semi-passive quiet mode with full fan stop for completely silent operation
Hardware switch to toggle between performance and quiet modes
6 optimized high-performance heat pipes
Optimized heat sink for more cooling power
Silent Wings 135mm PWM fan for perfect air pressure and virtually inaudible operation
Only 31.1dB(A) at maximum fan speed
High RAM and VRM cooler compatibility thanks to an asymmetrical design and cut-outs on the heat sink
Ceramic-particle black coating and magnetic top cover create the elegant signature be quiet! look
Compatible with liquid metal thermal grease
3-year manufacturer's warranty