Coolant leak/bubbling/whistling

New here! Will say right off the bat car knowledge isn't my strongest suit but I have been trying to tackle my latest problem.

About a week ago when parking up I noticed a puddle under my car (under the driver foot well it seemed to be coming from). Enough liquid to start trickling down the street. After looking under the hood I noticed my coolant tank was now near empty. So I re-filled it, with a coolant/distilled water mix and made my journey back. On the journey back I noticed a little bit of stream coming from under my bonnet and when I got home I realized my coolant tank was nearly empty again.

I re-filled it and sat on my drive revving my car, trying to heat it up and I noticed that my coolant tank was boiling, whistling, giving off steam and sure enough after a point water/coolant started spluttering out of the cap area. I thought it might be a dud cap so I bought a replacement one.

After replacing that I decided to re-fill it and make the short half mile trip to the petrol station to test it, I noticed some steam again, and this time I felt like I could hear bubbling coming from the tank. I also felt like the power steering got heavy turning corners, not sure how/if that's related but has never happened before. Sure enough when I got back the coolant tank was completely empty again.

I've checked my oil, there is no milky white coloration in the oil etc. There is no oil in the coolant tank either. I am wondering if replacing the actual coolant tank is the next step. It holds liquid absolutely fine when the engine is off, level doesn't deplete so wondering if its an expansion issue.

Any suggestions/diagnosis would be greatly appreciated as like I said, relatively clueless when it comes to this stuff.

Thanks guys.