The rattling sound you hear behind you is that of a wheel that has fallen off of a Hummer that's being towed to the salvage yard, never to return to the GM business plan:

...the collapse of the heavier-vehicle market in the U.S. because of high gasoline prices is pushing GM to completely juggle its business plan. The company has recently introduced a few passenger cars ... that are more profitable and sell at higher prices than typical GM passenger cars. Mr. Wagoner has said it needs to transform the entire car lineup into a profit center. GM decided to consider selling its Hummer brand earlier this year when the demand for SUVs began to significantly recede under the weight of $4-a-gallon gasoline costs. Hummers are widely viewed as gas guzzlers, and that image has crimped GM's ability to sell the SUVs amid high fuel prices. It has also slowed GM's push to convince U.S. buyers it is committed to fuel efficiency. People familiar with the matter said GM isn't any longer seriously considering a complete revamp of the Hummer division into a lineup selling vehicles with better fuel economy. That would be a costly proposition for the auto maker at a time when it faces a potential liquidity crunch. One significant hurdle for GM is its relationships with Hummer dealers. Because of tough dealer-franchise laws and other agreements the auto maker has with dealers, it needs to negotiate closely with hundreds of individual store owners on the future steps it takes with the brand.

The sound you don't hear coming up behind you is a car with nothing under the hood:

Lotus is developing technology that will put the roar of the traditional combustion engine under the hoods of eco-friendly vehicles, in an attempt to make the quiet cars safer for unsuspecting pedestrians – particularly the blind – and cyclists. Some are almost silent at slow speeds. Lotus said its "safe and sound hybrid technology" simulates the traditional grunt of a combustion engine, making it "instantly recognizable that the vehicle is in motion". It has already put the system into a Toyota Prius, one of the most popular hybrid cars on the market. The device kicks in automatically to produce an artificial engine noise when the hybrid car runs on its electric motor. When the car's combustion engine takes over, sensors fitted to the engine and suspension turn off the sound. The engine noise is produced by a waterproof loudspeaker positioned next to the car's radiator, making the sound seem to originate from under the bonnet. The system produces a pitch and frequency designed to help pedestrians identify the car's speed and distance.