Any clue what this part is got pic its above alternator w124
 

It keeps rattling and banging. 104 140 008

Running the complete part number 104 140 00 81 comes up with Cap , AIR pump.

This would be a bent driven air pump for emissions. If you are in a non emissions inspection state, you should be able to remove it and use a shorter belt.

Secondary Air Injection pump... it's an emissions reducing device and the bearings in them are known to fail causing lots of noise and eventually will overheat and seize, throwing the belt.

If your state doesn't require emissions tests then just throw it out.

cap rattle?
 

The part in the picture is just a cap over the outlet tube from the air (smog) injection pump. It is just a compression fit. See if it is loose by twisting it a little. This could be the source of your rattle. Just pull it off and pinch down the sides a little for it to fit more firmly. The air pump only comes on when the engine is cold and then turns off. When it is on, it does make some sound. If it comes on at other times or makes noise otherwise, it might be time to replace the whole unit.

Yea my car is making rattling popcorn sound because this is loose so just remove it then?

Ty

Well my belt slipped off and melted .leaving me stranded. Guess this part did something. Also pulley on the alrernator looks crooked. Wont spin

Is there a part number for this shorter belt?

I don't have one, it will be a trial and error thing. However running a string around the pulleys with the adjuster at mid travel will get you a # then look at the belt size catalog for something close.

Given the alternator won't turn, this is likely your original problem. Give everything a spin.

Yea the pulley is tilted no room to hammer it with radiator.

Belt got twisted around main pulley and fan melted on all the pulley groves. The alternator pulley is tilted crooked. Dont know if it needs hammer or remove whole alternator to fix. Smh last thing i needed

Please how do i get the pulley straight again or is it a bearing do i need new alternator

Hammering won't fix it, and is is surprising that the shaft bent. Pull the alternator and give it a spin to make sure the shaft is bent.

It's the air injection pump pulley is loose now as well as the big pulley with no teeth or grooves. My idler pulley half melted off.

I tried to tighten the air injection pulley but it just spins?

And the big pull keeps hitting metal and stopping when I spin it. All this because of that little cap..

Do I need the air pulley or can I get a new shorter belt and bypass it?