Play a deep tone in the car and find out what rattles.
A sine is the purest tone — recommended for bass testing. Square and sawtooth contain harmonics and are used to reveal distortion.
The WAV files contain exactly the tone or sweep set above, at the level shown. Uncompressed, so you can put them on a USB stick, cast them or stream them without a codec having touched the bass on the way — which is precisely what casting the browser tab does to it.
The sweep runs smoothly from the start frequency to the end frequency. Useful for finding resonances and bass nulls in the car.