Commuters on the A and C lines were subjected to rush hour subway delays this morning, which multiple conductors have apparently blamed on an "unusual noise" heard somewhere in the system.

The peculiar sound snarled service across Brooklyn beginning at around 9 a.m., with riders reporting that they were stuck for upwards of 20 minutes at Hoyt-Schermerhorn, Broadway Junction, and Nostrand Avenue.

"The conductor told us that there was an 'unusual noise' at Hoyt & Schemerhorn, but a friend of mine said her conductor on the C train said something was wrong at Jay st," Kimberly C told us. "I have no idea!"

Another commuter says that their train was stopped for what seemed like 20 minutes after leaving Nostrand Avenue. "Around the ten minute mark we were told it was an unusual noise. Some people were visibly frustrated. How will we explain this to our bosses? Some people laughed. What the hell is an unusual noise? You couldn’t hear the noise from where I was and they didn’t tell us what station it was in but I never heard the noise at any station we stopped at."

On Twitter, a spokesperson for the MTA explained that crews had heard the "loud noise" at Hoyt-Schermerhorn, then gone about inspecting each train car for the source of the din. They discovered that one of the train's motors had caused the noise, and while safe for now, it would have to be sent back to the yard for repairs.

The spokesperson also confirmed that the clammor came from one of the R46 trains, which have been plagued by various issues since they first arrived in New York City four decades ago—most recently, that harrowing incident on the F train back in 2017.

As of 9:30 a.m., A and C trains had resumed with residual delays, the MTA said. Meanwhile, northbound B and D trains were experiencing delays because of a customer in need of medical assistance at W 4th Street.

Anyway, while we're on the subject of loud subway sounds, why not add one more entry to your ever-expanding list of city-induced fears: According to researchers, the NYC subway system routinely exposes its riders to levels of noise deemed unsafe by the EPA and the World Health Organization.

Happy Monday!