Bad sound in car

[Sarcas]

Hi,

I'm using Spotify (premium) in my car, connecting with the lighning port to a Yatour iphone-kit system which I added to the simple build in radio of my car. The sound when using iTunes, TuneIn or another sound app is perfect. When using Spotify though the sound is very average. There is a lot of crackling going on. Not random, but in tune with the beat and certain tones, like the speakers have gone bad. 

- iPhone 6 plus, iOS 8.1.3

- offline music as well as online music, in any quality

- I tried removing and reinstalling Spotify

- disabled equalizer

- it is a fresh iphone, i just got it for two weeks

- not jailbroken, no weird apps running

Anyone advice?

thx!