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Low average interface utilization does not preclude there being transient congestion. (BTW, very low utilization can be a sign of very bad congestion.)
To also expand upon what was mentioned by supportlan, 3750s don't have a lot of buffer RAM (2 MB per 24 copper ports and also for the uplink ports), and the way 3750 allocates buffer RAM, by default, often leads to queue drops when QoS is enabled.
You mentioned enabling PQ fixed your VoIP drops. That's certainly a possible solution, especially for traffic in the PQ, but are the remaining queues still dropping? If so, at an acceptable level?
You mentioned, in your later post, you might disable QoS. That's also a possible solution, and it will often mitigate drops, but it leaves your VoIP traffic unprotected by QoS.
Perhaps the "best" long term solution, is to implement a QoS policy that meets your needs; taking best advantage of your equipment's capabilities.
On 3750s, I have had much success with changing buffer settings to mimic a common/shaped pool architecture. This works well when not all interface are busily transmitting queued traffic, but a few ports are dealing with a transient burst.
