A considerable increase of the noise contamination with the increase of the pulse-stream's bit rate in an optical fiber link with semiconductor in-line amplifiers is demonstrated experimentally and numerically. We propose a new explanation for the noise enhancement based on the amplifier's memory effect. In contrast with the pattern effect, known for the modulated train, the memory-induced noise enhancement manifests itself even in a uniform train. The analytical prediction shows a qualitative agreement with the numerical results. The memory effect is expected to be especially detrimental in the WDM transmission mode.