Apple’s 2016 MacBook Pro previously had trouble with its battery and now customers are complaining about another problem with the laptop. Some users are reporting on Apple support forums say they hear a “popping” noise coming from their MacBook Pro.

The sound is described by a user as a “single hollow" popping’ noise, like when slowly squeezing a plastic bottle.

“Sometimes I won't hear it for days and then all of a sudden I will hear the ‘POP’ 5-8 times over the duration of about 2 hours,” another user said on Apple’s support forum thread for the issue.

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MacBook users recorded the popping sound on video and uploaded it on YouTube and Vimeo:

Untitled from Skulcar on Vimeo.

MacBook owners believe the popping noise is related to thermal conditions.

One user claims to have found where the cracking and popping noise originated from:

“Pick up the laptop, with the screen open at 90 degrees and press right at the bottom of the screen assembly in the middle. It will crack and pop like a candy.

Now, I think the problem is actually from the hinges, as there is some natural flexing in the aluminum back of the display. I think the heat has affected the glue or some plastic mount in the display assembly.”

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Another user also reported a “hinge noise” issue, which makes a “clacking sound,” when opening the laptop for the first time after not using it for a while. However, the noise is not made when the laptop is closed. The certain keys on the keyboard on the user’s MacBook Pro also makes “loud-pitched noises” when the laptop is hot.

The user said:

“The computer makes the random popping sound every once in a while; to me it seems to be related to the device getting warm. It seems to happen much more frequently when I connect an external display (which causes the dedicated GPU to turn on and therefore increases the amount of heat). I can also (at least sometimes) reproduce the issue by running a program to get the CPU usage to 100%.”

One MacBook owner said Apple found the issue was caused by a small dent on the bottom casing. The user had to pay $130 to replace the entire bottom casing. Another MacBook owner took the laptop to the Apple Store and got the top case and battery replaced but the popping sound continued. The user then got a new MacBook Pro, but the noise was still there.

The complaints come after previous problems with the 2016 MacBook Pro. Last December, Consumer Reports declined to recommend Apple’s new MacBook Pro with Touch Bar citing “highly inconsistent” battery life tests. Consumer Reports rescinded its verdict on the product after Apple fixed the issue with a software update in January.