By Herbert Lash NEW YORK, Sept 22 - U.S. Treasury yields see-sawed on Wednesday after the Federal Reserve said it would reduce its monthly bond purchases "soon" and investors grappled with a timeline that suggested higher interest rates may follow more quickly than expected. The gap between five-year notes and 30-year bonds US5US30=RR fell below 100 basis points after the Fed statement to its lowest level since July 2020. A narrower gap is a possible indicator of factors like economic uncertainty, easing inflation concerns and anticipation of tighter monetary policy.