MacBook Pro owners on Apple’s official Support forums, as well as those on Apple-tracking site MacRumors, are reporting that their 2016 MacBook Pro models will at times make noises that sound like someone squeezing a plastic bottle. Others have simply said that they hear a “popping” or a “creaking” noise.

Exactly why the MacBook Pro is making such sounds is unclear, and although many people are saying they’re experiencing the problem in the forums, they don’t necessarily reflect the entire MacBook Pro user base.

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Apple (AAPL) released its MacBook Pro with a new featured called a Touch Bar in October. The Touch Bar sits above the keyboard and displays contextual information based on what’s on the screen. Users interact with the functions listed on the Touch Bar with their fingers. Apple touted the feature as the next big step in its computing evolution.

The forum commenters complaining of the popping sound all say that they own the MacBook Pro with Touch Bar, though they’ve bought the computer at different times and report different manufacturing dates.

The owners, whose comments were earlier reported on by Apple-tracking site 9to5Mac, also report different reasons for the popping sound. Some say that the sound happens for no apparent reason at random times, and others say it appears to pick up when the notebook is working hard on a resource-intensive task. All of the users say they can continue to use the computer with no apparent problems.

Still, several of those who posted to the forum said that they discussed the problem with Apple’s Support team. Different Apple Support people have suggested a variety of possible fixes, suggesting it’s not a known problem that Apple has issued a standard resolution on.

The popping sound, in other words, appears to be happening to a variety of MacBook Pro owners, but exactly why and how to address the problem are still unknown.

Apple did not immediately respond to a Fortune request for comment on the reported problem.