I have a 2015 automatic honda civic ex with 55k miles that's making a weird engine rattling sound sometimes when I put the car in drive. And when I turn on the A/C I hear the rattling sound all the time and it makes the noise for a few seconds and stops a few seconds and comes back, any idea? I checked my alternator and it has no problems. Heres a video of the noise.
When the AC compressor engages it puts a heavy load on the fan belt drive. Check all pulleys, including idler and tensioner. If the AC compressor itself is making that noise replace it ASAP before it seizes up internally, causing bigger problems (like having to replace the entire AC system).
Maybe "rig up" a megaphone to isolate where the noise is coming from.
Eric O demonstrates.
https://www.youtube.com/watch?v=fHtLH_m9gN4
Check the idler pulley
