My girlfriend has a 2006 Honda Accord LX 2.4L. She hasn't driven the car in about 2 years and when we parked it, it had a really bad clunk noise. I used a stethoscope and noticed the sound coming from the tensioner (noticed the casing had a cracked also) so I went ahead and replaced it thinking that was the problem. That didn't fix it. It has a bad clunk still coming from that area. I took the stethoscope to the valve cover and oil pan and didn't notice much of a clunk like I did when I put it on the tensioner. I can't pinpoint where it's coming from. Also, I revved up to about 3500rpm and let it drop to see what it did and it almost wanted to stall put but caught itself (dropped go like 400-500rpm). Any ideas on what it could be. I initially thought maybe the internals but the oil didn't seem excessively dirty or contaminated with metal. Car has 277k miles but always ran good and used full synthetic.
Heres a link to the video: https://youtu.be/lAwfITdYpcE

 

The belt is squealing pretty loud with a new tensioner. Is the belt damaged at all? If you remove the belt, try spinning the alternator pulley by hand and also the alternator pulley to see if they have any noise or excessive play. Inspect the crank pulley for any obvious damage. You may want to use a straight edge to see if one of the pulleys is out of line?