Brother MFC-L3770cdw printer is leaving black vertical lines on pages when copies are made through the automatic document feeder, but not when using the glass

Got it.
Let's determine if the black lines appearing on your documents are related to the printing or scanning area.
You will need to print a Test page.

1. Press Menu.
2. Press ▲ or ▼ to display Printer, then press OK.
3. Press ▲ or ▼ display Print Options, then press OK.
4. Press ▲ or ▼ to display Test Print, then press OK.
5. Press Start. The machine will begin printing the Test Print page.
6. When finished, press Stop/Exit.

Keep me posted on how it is going, please.

So it's not as clean, right?

Could you please specify one thing for me, are there black verticle lines on it?

Got it, thank you for cooperating with me!
In this case we need to perform a manual drum cleaning.
The items required for this cleaning are:
- An alcohol swab, such as a medical alcohol pad or wipe, which can be found in a local retail pharmacy or first aid kit.
OR
- A cotton swab lightly moistened with isopropyl alcohol.

I think it's okay.

Sure, take your time!

So let's get started.

• Open the front cover and remove the drum and toner assembly.
  - We recommend placing the drum and toner assembly on a piece of disposable paper in case you accidentally spill or scatter toner.)
• Push down the green lock lever on the left side of the drum unit and toner cartridge assembly and then lift the toner cartridge out of the drum unit.
  - (Handle the toner cartridge carefully. If toner scatters on hands or clothing, immediately wash it off with cold water.)
• Turn the drum unit as shown in the illustration. Make sure the drum unit gear is on the left side.
  Let me know once it's done please, so I will send you the next steps.

Step 3 should look like this:

• Turn the gear at the edge of the drum unit toward you by hand while examining the drum surface for any marks or foreign material. Turning the drum gear will ensure you inspect the entire drum surface. When finished inspecting the drum surface, turn the drum back over to its original position.
• Inside the drum unit is a roller. Clean the roller by gently wiping the entire surface with a lint-free cloth or cotton swab lightly moistened with isopropyl alcohol. You can also use an alcohol swab, such as a medical pad or wipe. Use the white gear on the far left side to turn the roller and ensure you clean the entire surface.
  Like this:

• If physical damage such as scratches, nicks, or gashes are found in the surface of the drum, there's no need to continue cleaning since it won't resolve the issue. The drum will need to be replaced.

The drum is combined in the same cartridge as the toner.

Got it.

• Before continuing further, allow the alcohol the completely evaporate or dry off the drum surface.
• Once the drum surface is dry, clean the primary corona wire inside the drum unit by gently sliding the green tab from left to right and right to left several times.
  -Be sure to return the green tab to the Home position (▲). The arrow on the tab must be aligned with the arrow on the drum unit. If not, your printed pages may have a vertical stripe. Like this:

• Verify that the green tab on the corona wire has been returned to the Home position (▲). If the arrows are not properly aligned, this will cause pages to print with a black line.

Oh, now there are no lines?

Amazing!
I'm so happy I was able to help you.
Is there anything else I can do for you today?