Hello - my mom drives a 2006 Lacrosse CXL with the 3800 V6. She purchased the car two years ago with only 58,000 miles on it. It now has 70,000 miles on it. Previous owner was an older woman who wasn't able to drive anymore, but the car was well taken care of. Last week she said it started making a knocking noise when driving, which gets louder when the air conditioning is turned on. She took it to a local mechanic who confirmed there is a knocking noise coming from the lower end of the motor. He removed the accessory belt and the noise is still present, but not as loud as when the air conditioning is turned on. He is adding an oil additive to see if it helps with the issue, but said if it doesn't it would like require the lower end of the motor to be torn apart to troubleshoot further. I have heard these 3800 V6 motors are ultra reliable. Any thoughts on what the issue could be?

Have you had a Check Engine Soon light on at all? Or any ding ding sound from dash?

Before going into the lower end of the engine, I would take a very good look at the harmonic balancer. I understand that the serpentine belt was removed and that was a good procedure but would go a bit further with inspection and testing since we know the 3800 is super reliable. Also, if the noise is always the greatest when the AC is on, I might focus on the AC compressor/clutch with further inspection and testing. The mechanics stethoscope may be needed here.

Harmonic balancer - that makes perfect sense! First thing that pops up when I google 'Buick 3800 Harmonic Balancer' is this video: