I took my car in for inspection, and it failed due to the right lower control arm bushing being cracked.

They replaced the entire lower control arm, got it aligned, and it passed. However, I've noticed that between 20-30 mph there's still a slight vibration. I called the shop and they said that there weren't any faults in the motor mounts (which are a part of state inspection), and that although the tires were starting to wear down a bit, I'd be fine for the next few months.

I'm at a loss as to what's causing the rumble now. Could it possibly be the steering column?

 

I'd look at tires & wheels - seems more likely than the steering column.

Try rotating the tires to see whether the vibration seems to move to a different corner of the car. You can have the wheels checked for out-of-balance. You can check for bent wheels yourself by lifting each corner & spinning the wheel.

If it turns out to be cupping or other kind of funny wear on the tire treads, you can decide how long to tolerate it before replacing the tires. If a sidewall is bulging out funny, then don't wait for it to blow out.

Back to the steering...
Lift and support the front tires off the ground. Have someone reach in through the window & hold the steering wheel from turning. Grab each front tire & try to "steer" it side-to-side. Shouldn't be any looseness or knocking.

 

Axle(s)

 

Axles commonly have vibrations while you accelerate; less or none when coasting.

If the vibration behaves like that, it's likely the axle. If it doesn't behave like that, it doesn't really rule out the axle either.

 

Thanks for the replies! I can confirm that the vibration only occurs during acceleration at low speeds, and not coasting or decelerating.

With my next check, I'll take it back to the garage and have them look and see if the wheels are out of balance. I'll also have them take a look at the axles then report back.

 

Does not indicate any kind of a balance issue.

Moving directly the an examination of the axles is a better idea.

 

Since it only happens during acceleration, I'd go right to inspecting the axles.

Out-of-balance wheels would do it during coasting & deceleration also.