Why would my new 2015 Macbook pro make more fan noise than my 2013 model?
There's nothing about i7's in particular the determine that they should always lead to more fan noise than an i5. In many cases, i5's and i7's have similar thermal design power points.

Note that you're comparing an 2013 MacBook Pro with an i5 with a 2015 MacBook Pro with an i7. They're different in many ways. Therefore you cannot really say anything meaningful about whether it is due to an i7 or some particular other thing.

However, if you're annoyed by the noise, I would suggest cleaning the fans. Your laptop has potentially seen 4 years of use in an environment you do not know. The fans could be very dirty and that could in itself lead to a lot of extra noise. It might also be possible to reduce noise by reapplying cooling paste to the CPU.