Your question: "Ok I have a Nissan Altima 2003 2.5 sl. A couple weeks ago there was a hissing sound coming from the car when I accelerated to around 40. And now it's at the point at when I accelerate at any time I hear hissing. This hissing is very very loud. It's sounds like it's coming from both sides of the car can't really tell what it is.
If you're by the car and you can't tell where the noise is coming from, you know we can't tell, but if it's loud, it is not likely a vacuum leak. It is more likely an exhaust leak. You need to have a mechanic inspect the exhaust system to determine the cause and fix. It could be as simple as a deteriorated gasket at a flexible joint. It could be a rusted joint between two pipes or a crack in a pipe. It could be as serious as pinhole leaks in all of the pipes and muffler.
Professionals will inspect the entire system, and they will not replace just the worst part and leave other leaks in the system. One thing to be aware of is it can be difficult and time-consuming, meaning expensive, to try to remove a leaking part from a good part it is connected to, so to save you money, most of the time both parts are replaced. For that reason, for example, one mechanic might tell you the muffler is rusted out and needs to be replaced, and a different mechanic gives you an estimate to replace the muffler AND the tail pipe connected to it. Saving the 20-dollar tail pipe might require a half hour of work and cost 40 bucks. You'd spend more money and still have an old, rusty tail pipe.
On the other extreme, another thing to be aware of is a lot of exhaust specialist chain stores make their money on selling parts, and they usually end up costing you a lot more money. Since exhaust work is all they do all day, they will get the job done faster, and there's nothing wrong with the quality of their work. There's just a much better chance that you'll end up with more new parts than you might have needed. All of those pipes are the same age, but those near the front of the car stay hotter and moisture boils out of them faster so they take longer to corrode through. Mufflers and tail pips rust out fastest. If those are what's needed, a reputable shop will not try to sell you other parts. Most of the mechanics at the chain stores work on commission, so they have an incentive to sell you more parts.
Along the same lines, it is common to need new hangers with the new pipes. The old ones rust out faster than the pipes, and even if they aren't rusted yet, trying to reuse them is another example of spending more in labor cost than what the new parts costs.