I can't find any sources detailing this conflict from before 1929 and those sources say they were diplomatically friendly. Are there any sources i could look at that detail what their previous relationship was like and when said relationship began.
The tension between Iran and Saudi Arabia started after the Iranian Revolution, and it’s really because of two major factors.
The first is post-revolution Iran’s strong anti-American sentiment, which included opposition to the American bloc, of which KSA has been a key part.
The second is the tension between Saudi Wahhabism and Iran’s shia nationalism. Shia communities all around the Middle East, who were mostly either leftist or non-political prior to that, quickly embraced the new wave of religious shia nationalism. You can see this as analogous to the USSR and communist/socialist parties all around the world.
The latter factor continued to cause incidents. The shia minority in Saudi Arabia had a violent uprising immediately after the Iranian Revolution, and it has gone through waves of instability ever since. Most recently in 2015, the execution of Saudi shia clergy Nimr al-Nimr lead to riots in Tehran and the Saudi embassy was set on fire. That’s just the most obvious example.