I Hear You Knocking

 is a quest in Fallout: New Vegas.

Quick walkthrough
Lucius wants you to get a replacement firing mechanism for the Legion's howitzer from the Boomers in Nellis Air Force Base.

Quest Start
You access this quest at The Fort after you talk to Caesar, speak with his subordinate Lucius. From there talk to him until you reach the question about whether or not Lucius has employment opportunities. He does. Caesar's Legion recently obtained an artillery weapon (which is aimed currently at the hoover dam), but they do not have the part to fix it. Then he tells you about a tribe of people called Boomers who are obsessed with such weapons.

Get a Replacement Firing Mechanism for the Legion's howitzer from the Boomers
In the northeast corner of the Mojave Wasteland you will find Nellis Air Force Base, and after some careful running and dodging artillery fire you will be stopped by a boomer guard. Then if you agree to go willingly with Raquel she will take you to the eldest of the tribe Mother Pearl. Talk to her, you can ask her for the firing mechanism, but she won't be so willing to part with it. After questioning you realize that you are going no where way to fast, but now you have free roam of the base. There is a way to get the mechanism without stealing it. Become accepted by the Boomers and then ask Mother Pearl for it.

Retrieve the Firing Mechanism from the Boomer's Workshop
Once you exit Mother Pearl's barracks head for a row of curved corrugated buildings, enter the second building from the end. Once inside, look for the Gun cabinet in the corner, open it and among other items you will find a Howitzer Firing Mechanism. take it and return to The Fort.

Install the replacement firing mechanism into the Legion's howitzer
When you return search out the Howitzer on the cliff. Examine the Howitzer and replace the firing mechanism.

Inform Lucius that the howitzer has been repaired
Return to Caesar's tent, and inform Lucius that the howitzers have been repaired. You will gain Caesar's Legion fame as well as 100xp.