Equinox

I have a Tandem with a triple. I had a new chain put on by my local bike shop. On our first ride we encountered a slight grade. We were in the middle ring, in the middle of the cassette. There was a very loud noise from the back and I got the feeling like i couldn't turn the pedals. I was able to unclick. Everything looked OK. It didn't appear to be auto-shifting. So we continued our ride, and anytime we put pressure on the crank in this gear combination, we got this same awful noise. A new cassette did not make a difference. I took it to a different LBS because I was upset at the first LBS for other reasons. They saw the derailleur being pulled up on a test ride. It doesn't do this in the large ring. Just middle ring-middle of the cassette. The new tech thinks something is wrong with one of the pulley gears. I told him to put on a new RD, but he is going to take the old one apart to try to figure it out. I didn't have this problem until the new chain was installed. Could it be related?

FBinNY 

Odds are that the B-screw is misadjusted, especially if it's a Shimano derailleur. Most shops set the B screw using the larger chainring, to bring the RD as close to the cassette as possible, without actually touching it. Then they omit the next step in Shimano's instructions which is to shift to the smaller chainring and double check for clearance in all gears.

Usually the RD needs to be lowered slightly because on Shimano and Campagnolo derailleurs the upper pulley is above the cage pivot and rises was the cage rotates back to take up slack. A high derailleur allows the pulley to engage the cassette through the chain and can cause the RD to be torn off the bike.

The correct B-Screw adjustment calls for 1" of chain free between the pulley and sprocket so the RD is isolated from the sprocket can't catch the pulley. This also gives the best shift performance.

If the B-screw isn't adjusting the height properly, look behind the RD and see of the stop tab, or screw is properly engaging on the stop at the bottom of the hanger. Sometimes a mechanic mounts the RD improperly, trpping the tab or screw between the hanger ad upper body.

Equinox

The tech did mention a B-screw adjustment, but apparently it did not work. He did not mention anything about the tab or the screw being trapped.

FBinNY 

You don't need the mechanic for this. Download the instructions for your derailleur from the make's site. It's very straight forward. You'll refer to them for more detailed instructions, but here are the basics.

1- test the stop tab, by pulling the RD back against the chain tension as you would to remove the wheel, then swing it forward and it should come to a hard stop (warning, this doesn't apply to various derailleurs, including Campagnolo since 2001, with upper body springs).

2- adjust the B-screw (behind upper body) so the RD is close to the cassette, leaving 1" of free chain between the upper pulley and each sprocket (test one by one, using all chainring sprockets).

Also, while you're there, back pedal and check for stiff links on the chain, and that all else seems OK.