My son-in-law runs a small repair garage and has this tool called chassis ears, which allowed me to diagnose a 2 month infant mortality problem on my subaru rear wheel bearing. This tool can process up to 6 channels but comes with 4 channels of ears. The ears consist of a transmitter which attaches to a cable with a rca jack. The cable has a medium sized clamp like a booster cable clamp, with a built-in microphone. It can be clamped to a bolt head in the hub and the transmitter can be anywhere within a couple of feet. The transmitter attaches with a multitude of clamps or velcro strips. On the inside of the car, the base unit has selectors for up to 6 monitors. The base unit has a speaker and a headphone jack. I fitted up my car with 4 channels, one at each wheel. I grabbed my earbuds and hearing protectors and set off. It was instantly confirmed that the 2 month old wheel bearing had died, when I expected that it was time for that bearing's axle mate to be due. The difference in sound was unreal. I was also surprised at how sensitive it is. You could hear the brake rotors rotating and gently brushing the pads. The best part of this tool is that it allows the car to be used in normal driving instead of on a lift, or with a wheel jacked up. Mac tools puts this thing out. It is an amazing tool.
MacTools UK | WIRELESS CHASSIS EAR