Intermittent sink drain gurgle, but no slow drainage

My bathroom sink has started making an intermittent gurgling sound while it drains, although the basin still empties at its normal speed. There’s no obvious odor, bubbling, or visible movement in the water after the stopper closes.

Before opening the wall, I’m trying to narrow down whether this is more likely to be:

- A partially restricted drain line
- A blocked or poorly functioning vent
- An incorrectly installed P-trap, especially if the trap arm is too long or not properly sloped

The noise seems to happen during sink draining, not when the toilet or shower runs. The trap under the vanity is a standard-looking P-trap, but I haven’t confirmed whether the trap arm or vent connection is configured correctly. There was plumbing work in this bathroom previously, though I don’t know exactly what was changed.

Would you start by checking the stopper and trap for a partial obstruction, or is the timing of the gurgle more suggestive of a venting issue? Are there safe observations, such as temporarily removing and cleaning the trap or watching the water level, that would help distinguish those causes without using drain cleaner?