CRJ-900 Flight Attendant reported an air leak from the L1 door after takeoff. After reaching 10;000 feet; The Flight Attendant reported the door leak to The Captain. The flight crew elected to perform an air turn back and precautionary landing at departure airport.

Narrative

Aircraft X (ZZZ1 [to] ZZZ2) took-off from ZZZ1. I was working the forward FA (Flight Attendant) position; seated in the jump seat adjacent to the Main Cabin Door (MCD.) About 3 minutes after takeoff; approximately 1 minute before receiving the double chime indicating that the aircraft reached 10000 ft.; I heard a slight popping sound followed by a very loud whistling sound coming from the MCD. In my experience; this was a highly unusual sound; very different from the normal air noises that come from the door.Immediately upon reaching 10000 ft.; I called the flight deck. The Captain could hear the sound over the interphone and asked me what was that whistling noise; even before I had the chance to tell him. I explained the sequence of events to him and he said he would look into it and contact me back. A minute or two later; he did call back and told me that they were getting a reading in the cockpit that indicated low cabin pressure. He said they had leveled off at 10000 ft. and we're going to contact Dispatch about returning to ZZZ1. He then made an announcement to the passengers and informed them. I made a follow-up announcement to inform passengers that we were going to suspend service. Both FA's walked through the cabin to ensure that passengers were okay and to alleviate concerns and answer questions.After several more minutes; the CA (Captain) called us again with an update: we were currently in a holding pattern to burn off enough fuel to land. He told us that we did not have enough fuel to fly ZZZ2 and [had] too much fuel to land. He indicated that we were almost ready to start our approach and to prepare the cabin. He then made an announcement to the passengers with a similar update. We prepared the cabin; performed normal safety compliance checks; answered as many questions as we could; and returned to our jump seats for landing.After returning to ZZZ1 and arriving at the gate; we deplaned and informed [the] Passengers to stay near the gate area for updates on whether we were changing planes or not. Ultimately; we did swap planes and left ZZZ1. We arrived in ZZZ2 without any further incidents.