'98 is definitely EPS and I don't think it suffers exactly the same end wear problem as the manual rack or have the same repair. On the manual rack the noise would appear on the passenger side because the passenger end of the rack develops wear because there is actually no dedicated bushing there. The repair involves installing a bronze anti wear bushing. Details here
Hi, guys here in the land of Oz we have engineered a solution to eliminate steering rack knock experienced by older and/or high km/mileage NSX’s by using the NSX Knock Bush. What we have found is the rack knock is caused by the rack housing on the passenger side wearing - there is no bearing or...
www.nsxprime.com
If you can hear the clunk right 'at the wheel', I am more inclined to think a tie rod end joint. You should hope that also because that will be a cheaper / easier repair than just about anything else. Jack the front end of the car up and remove the wheels. Place one hand on the steering knuckle where the tie rod attaches and the other on the very end of the tie rod. Have somebody gently rock the steering wheel back and forth. If there is wear in the tie rod end joint you will be able to feel the slop as a small movement between your hands. Obviously check both wheels. If only one tie rod end is worn replace both because at 26 years of age, the second one is probably not far behind.
If it is not the tie rods, then put the wheels back on and grab the tires at 12:00 and 6:00 o'clock and try rocking the tire to generate the noise. Repeat at 03:00 and 09:00 o'clock. This rocking may show noise if its ball joints or A arm pivot points.
If it is not the tie rod ends or suspension pivots / ball joints then the problem may be associated with the rack. The rack is mounted in rubber bushings (8, 12 and 13 in this diagram)
1997-2005 Acura NSX Rack And Pinion Assembly 53601-SL0-A04
acura.oempartsonline.com
As they age and harden they may be allowing movement of the whole rack and be making some noise. But, I expect that noise would materialize more as body noise rather than a noise at the wheel. Based upon your description I don't think this is the issue; but, rule it out because the final option is more time consuming and expensive. If it is a rack internal issue, your best bet is removal and shipping to NSX Rack Repair
https://www.nsxrackrepair.com/