Will a Be quiet pure loop 2 120 mm cpu cooler work in a dell inspiron 3670

Not certain, but as long as manufacturer provides all needed brackets for LGA 1511 and fan cables, it should work. 

You can review this upgrade video for anticipated processes with installation and a known upgradable cooler.

3670

the oem board is mounted on fixed M3 studs of chassis floor, so a non-Dell cooler that does not use M3 backplate would not work (unless you use a plier to twist off those studs which I do not recommend)

https://www.youtube.com/watch?v=M8P1Tw4P-7s

playtime 12:40

you could use Dell premium tower cooler for Optiplex 7080 but it may be taller than the 3670 case width, which means in that case you can not close the side panel.

7080

you can look for a non-Dell tower air cooler that uses M3 backplate and low profile,  you do not use the new cooler backplate.  rather you just mount it on the Dell studs (M3).

7080 tower cooler

if you plan to use a 120mm AIO liquid cooler, one that works with Dell M3 is Corsair H60.  user has validated it in XPS 8940.  it works with Dell M3 studs.  I am not sure how you would connect liquid pump power in 3670 though?  do you connect it to cpu fan header?  the pump may need more power/current than what a fan header can supply safely.  you still need a radiator fan header too.  it is probably easier to use tower air cooler.

Not certain about Be Quiet Pure Loop cooler but I posted a YT link to Asetek 545LC 92mm AIO upgrade.

this may be similar to chino's YT

Inspiron G5 5000 user was able to upgrade to Asetek AIO

Asetek 645LT 92mm AIO

G5 5000 also has the M3 studs of chassis floor and no backplate fixed to backside of oem board, similar to 3030

https://www.dell.com/community/en/conversations/inspiron-desktops/liquid-cooled-g5-5000/647f9306f4ccf8a8de4fe6e0