Pump started to make a rattling noise after 4 yes of perfect operation.

start the engine up and keep an eye on the power steering pump. if you see the pulley wobbling, shut the engine off and replace the pump. I dont recommend reusing the same pump because if it is wobbling, then that wobble will damage the shaft seal. another highly cause of that rattle might be just an exhaust shield, as you drive the exhaust expands with the heat and the noise goes away.

The technique I always used to identify where such a noise was coming from was to use a very long screwdriver (2 feet long), while the engine is running. CAREFULLY touch the screwdriver end to each pulley or other object under the hood, and put your ear to the handle. The device making the noise will become very clear.

It could be AC pump/clutch, alternator, water pump, power steering pump, etc.

Yes time for a new ac pump it is wore out. Sorry for the bad news but the ac pumps dont cost that much.

check power steering pump valves as the noise could be valve vibration from weak control springs
timing chains do not rattle it properly adjusted and the so called rattle could be the injectors/ needles operating correctly

Hey Jean! You may want to make sure the dust cup is inserted correctly and the brushes are clear of hair.

If you continue to experience a rattling noise, you may want to have a technician at one of our Authorized Service Centers inspect your machine.

Here's a quick link to locating one.

http://www.bissell.com/service-centers/?cid=ccsocial

Yes they are prone to piston rattle and rocker arm noise until the lifters pump up. These types of noises are nothing to worry about unless they don't go away after a few minutes of run time.

READ THIS (happy ending so far...):
audi a4 2L turbo quattro, manual transmission with 98,000 KM. Perfect conditions, no oil leaks whatsoever.

Left the car off for two weeks in very cold weather (-10C down to -25C). When I restarted, it started immediately but with heavy rattling noise due to camshaft adjuster not getting enough oil pressure. After 3 minutes of warming up (outside temp.: -12C) at very low rpm (below 2,000rpm), I start moving the car. Still hearing heavy rattle noise. As soon as I pass 2,000 rpm, I get a scary red warning, low oil pressure. Turn off the engine immediately. In 1 minute, I re-start the car and the engine lights check ok, so I drive in first gear super slow (1,200rpm) hearing the same rattling noise. After another minute, finally the noise all the sudden disappears and the car starts running just fine.

Called Audi dealer and they told me that in extreme weather conditions, I should warm the engine for 10-15 minutes if necessary before start moving.
Take away: luckily, I didn't screw up the engine since I was using it at extremely low rpm (easy to do with clutch and manual transmission) until the oil reached full pressure. I am pretty sure now that the problem was leaving the car off for such a long time in freezing weather. All the oil flows back to the bottom, and when I restarted it, the cold oil with high viscosity took a long time to reach all the parts of the engine, specially the camshaft adjusters that are actuated by using oil pressure. Next day, the car started at -9C perfectly without making any noise (as usual). Basically, the longer you leave your audi in cold weather, the longer you might hear the rattling noise until the oil reach full pressure inside the engine. My car took 6 minutes to fix it after couple of weeks in cold weather. This makes sense to me; in fact, in the past I only heard the rattling noise after leaving the car off for a couple of days in the winter.
So far, this has had an happy ending....

yes if the oil pump shaft or balnce shaft are off it will cause a rattle

I didn't get to answer this but It makes "cents" to me.

Ensure that the water pump is turned off then locate the water pump in your RV. Typical locations are under the dinette, under the bed, or in a cabinet near the sink. If you can't find it, turn on a faucet and locate the noise coming from the pump (this should be really easy!). A typical water pump will be secured by 4 screws. Remove these 4 screws and slip a rubber mouse pad under the pump. Re-secure the pump being careful to not over tighten the screws. Using a mouse pad will help absorb the vibrations from the pump to the floor. Next, we will locate the rattling water pipes. Turn on the pump. Have a friend open and close any faucet while you try and locate where the clanging noise is coming from. Typical areas are where the cold water pipe passes through or runs along a cabinet wall. In this case, the cabinet acts like a sounding board and creates a lot of noise. Take some 1/2" foam pipe insulation and wrap the pipe where it rattles against the cabinet or flooring. It should slip right over and not require any securing to the pipe. But a little duct tape on each end of the insulation wouldn't hurt. Repeat this step until you have insulated all rattling pipes. Sometimes the water pump can rapid cycle on and off (cycling ON/OFF in less than 2 seconds) during low flow conditions. This can cause pulsations in the water system and create noise. To determine if adjustment is necessary, turn a faucet ON to lower than average flow of water. The pump should cycle, but its "OFF time" must be 2 seconds or longer. If the cycling is correct, no adjustment is necessary. If the pump is cycling rapidly, increase the setting by turning the adjustment screw (see image) clockwise (1 turn MAX.) until the pump operates for 1 sec. with at least 2 sec. "OFF time".