New owner with some questions

I'm a new owner of 2010 Prius trim II for only 4 days and very happy overall as the car is much better compared to most reviews about the Prius.

The first 2 days of driving gave me low MPG starting initially at 25 MPG and finishing at 37 MPG. The last 2 days gave me about 50 MPG. I guess this normal for a new car to start with poor MPG, but the difference surprised me. I wonder if anyone had similar results. The Prius had only 7 miles on odo and was only 3 months old based on the production date.

One more question, the engine compartment makes some noise every time I open the door before a drive before the car is started. I did not notice that on a test drive. I assume this is normal, but what is it?

Finally, I need to say now it's great time to buy them. I scored over 3,000 below MSRP and 2,000 below invoice on a base model. The salesman was desperate to sell anything one hour before closing time last day of the month.

My plan B was used but clean 2010 with 30,000 miles I saw on ebay (Houston Toyota dealership). The buy it now price was $18,000 and no one bought it.

 

Ken, I read the manual cover to cover on the first day. I did not think this specific noise was mentioned or explained. I went back to the page 31 and nothing is mentioned about opening driver's door nor pressurizing brakes. The manual talks about "sounds from hybrid system" while opening "back door" (whatever that means). Thanks for your help though.

 

Thanks!
BTW, I never noticed a braking issue after start in my previous "normal" cars. Maybe the vacuum brake servo leaked in your PT cruiser?

 

Thanks!
One thing I neglected to mention that the tires in the new Prius were pumped to only 30 PSI, but had "N2" valve caps! I added a precious mixture of 78% N2 and 21% O2 to 42/38 PSI. But I did not think this would explain the huge difference in MPG. Maybe extra gas went in charging batteries that were lowish from the beginning?