RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes
The funny noise probably is from high order harmonic sound waves and could be relatively high pitched (see [link=http://www.rcgroups.com/forums/attachment.php?attachmentid=2815700]this URL[/link]). The cited increase of noise level could be 30 dB at the axial direction. Decreasing RPM probably is a good idea to conceal the extra noise out.
Since the detail of the drive has not been revealed, this question may be ir-relavent. Would it be possible to try different combinations of aft and/or foreword props on the noise etc. design parameters?