Techs new to scopes can often try to read too much into the pattern on their screen, finding faults where none exist' and taking off onto wild electrical goose chases that waste their time. This can be especially true when using a modern, high powered, scope that can pick up the smallest electrical signals.
Consider this pattern...a CAN C bus signal taken directly at the DLC (Diagnostic Link Connector). See the "hash" throughout the pattern? If you were diagnosing a bus communications code, would you worry about this "hash"? Could it be the indication of a problem?
It might, if you didn't know what normal interference on your scope looked like. Here's another pattern showing the interference caused in my shop by something simple...the overhead lighting. My leads are shielded, but when a dolphin clip is added to the positive lead it acts like an antenna, picking up on the magnetic field caused by the lights. With the clip removed, the effect is reduced. The leads are NOT connected to anything at this point.
Notice the relatively small amount of voltage induced, and the repetition of the pattern. Vehicle induced EMI can cause this interference as well, and typically results in a repetitious pattern that matches the operation of the circuit causing the interference...an ignition event, for example. The under hood environment can be a noisy place electrically. Be sure to use shielded leads and know what noise looks like on your scope to make sure you don't end up chasing "ghosts".