3:37 PM PT -- Cops are on the scene, and according to a neighbor it appears Aaron is fine. He was seen answering the door for the responding officers.

No word on whether he was under the influence, as the concerned caller claimed.

Cops are rushing to Aaron Carter's home after getting a report he appeared to be under the influence of something ... TMZ has learned.

According to law enforcement, cops just got a call from someone who had been on a call with Aaron when they felt something just wasn't right. Sources connected to Aaron tell us one of his neighbors went to check on him, but no one responded when they knocked.

Police are en route right now.

As we've reported ... family members have been worried for a few weeks now that Aaron was relapsing and possibly suicidal. Last week in NYC, Aaron told us ... there's no reason for concern.

Story developing ...