Any Unknown or Unlisted Aircraft Manufacturer
Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine
Flight Crew Last 90 Days 200; Flight Crew Type 19000
Confusion
ATC Issue All Types; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Weather / Turbulence
Air Traffic Control Issued New Clearance; Air Traffic Control Provided Assistance; Flight Crew Became Reoriented; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Returned To Clearance
Chart Or Publication; Human Factors; Procedure; Weather
We were in IMC near thunderstorms and the conditions were causing a loud static noise on our radios. Even with the radio volume turned up; it was difficult to hear. We were given another amended route clearance. The Controller spelled the first fix phonetically LEEDN and then the GOPAC 2 Arrival. Because of the poor radios; I wasn't sure of the last letter of the fix. A quick glance at the GOPAC 2 chart showed the fix LEEDR; which was just a few fixes east of our original clearance. I assumed that was the point we were cleared to. I thought I read back LEEDR to the Controller. We programmed the FMC to go to LEEDR and were given a frequency change to Chicago Center. Upon check-in; I told Chicago we were direct LEEDR. A few minutes later; Center advised us that it looked like we were left of course and asked us if we were going direct to LEEDN. That's when we noticed LEEDN was a fix on the southern branch of the arrival. We reprogrammed the FMC and then flew to LEEDN to join the arrival.
An Air Carrier Captain reported they misunderstood their clearance and navigated to the wrong fix. The RNAV STAR has similar sounding and spelled fixes named LEEDR and  LEEDN.