flapjax

1998 Subaru Outback 2.5 with 135K miles. Motor replaced at 42K. I recently had the 120K service and the timing belt/tensioners replaced at a local "japanese specialist" garage that has good reviews. While driving home from the shop, I noticed a noise that keeps time with the motor's rpm. It reminds me of the sound of a playing card slapping against the spokes of a bikes spokes. My initial thought was it might be bearing or valve noise. This noise did not exist before the local shop did the work described. I took the car back to the shop and had the service manager listen to the noise. He acknowledged that there was a noise, but declined to comment on what it may be. I again reminded him that the noise did not exist before the service. He called later and stated that they checked the timing, compression, and anything else they could think of but could not determine what the noise was. He informed me that they felt confident that the noise was not cause for concern... they did not think it would cause any future problems. I drove the car for a day, and when I took it up to 65 mph and accelerated, the noise seemed to become worse. I also notice it every time I accelerate during daily driving. I took the car back to the shop, and asked them to "make things right" since the noise originated after the work was performed at their shop. They assured me they would, but called later and said they could not replicate the noise I described at 65 mph. They also felt that the car was running well. Any thought on my next move? I feel that the motor is slightly less responsive than it was before the service, though it does idle more smoothly. Thanks for any suggestions or insight.