A driver has complained of a loud, bumping noise in the area of the rear cab mount when traveling on rough roads. The most likely cause of this noise would be a/an

• A. Loose cab mounting bolt
• B. Worn shock absorber
• C. Deflated cab suspension air bag
• D. Damaged leaf spring

Answer: (C)

The most important idea here is how a cab suspension air bag failure changes the way the cab rides and sounds over rough pavement. The cab suspension air bag acts like a spring that cushions the cab and keeps it at the proper height. If the air bag loses pressure (deflates), it can’t support the cab weight properly. The cab then sits lower and can’t absorb bumps as well, so every time the wheels hit a rough patch, the cab tends to bottom out or shift sudden­ly against its mounting hardware. That metal-to-metal contact at the rear cab mount makes a loud, banging or bumping noise on rough roads.

So, a deflated cab suspension air bag is the most likely cause because it directly reduces the cab’s ability to smooth out vertical motion, leading to audible impacts at the mount points. By comparison, a loose cab mounting bolt would usually produce a more general rattle or clunk, a worn shock absorber mainly causes increased bouncing rather than a sharp bumping sound at the mount, and a damaged leaf spring tends to change ride height or cause other symptom patterns rather than a focused bumping noise at the cab mount.