I'm outputing signal in arduino in pins 3 4 5 6 7 8 one after another with a specific time delay
I'm sensing the output voltages, and I see that each pin feels kinda noise (a little of the signal that the next or previous pin output)when the other pins are outputing
is there anyway to "clear" that noise?
There will always be noise. However, it is rarely (if ever) a problem, as the "static discipline" of the digital communications is designed to take that noise into account.
Where are you seeing the noise? Typically logic signals are pretty clean if not running parallel to other signals for some distance - oscilloscope probes often pick up far more noise than the logic line they are attached to as they are not transmission lines and inductively and capacitively couple to nearby signals. A proper logic probe is a low-impedance matched transmission line that more accurately reflects the true signal.
And as has been said 0.5V of noise is irrelevant on a 5V CMOS logic device, its within the noise margin.
Another plan is to either terminate or drive high or low any unused pins. Analog measurements often fail, in that they are frequently 'noisy' this is because the analog pin isn't being driven properly. Most of the "design" work I see done here is simply "Hook it up" and hope it works... Proper bypassing, the use of a Star ground configuration and returning analog grounds to a point closest to the analog ground of the '328 or other device (Mega is a great one for that). The single thing ithat everyone (almost) disregards is that EVEN though the Arduino is capable of Amazing feats... It IS a TEACHING Platform... Not a general purpose I/O Platform for industrial and semi industrial use. Because of that "short' coming many projects fail to one degree or another because the things necessary for an industrial controller are NOT required for a teaching platform. I wish I had a nickle for every sketch where extra work spent in "Conditioning" poor input data due to platform limitations was spent in "fixing" a board layout not intended to be an Analog data logger, or a temperature controller... or for "Fixing" "Crosstalk" that in fact is due to a lack of bypassing, poor power supplies (9V batts driving motors) and other failings. It isn't enough to be a great "Coder" If the Code can't run on the platform due to the limitations of the platform. In order to make a sketch work it is very necessary to understand the hardware that is being directed by the sketch as much as what the sketch is really supposed to do. 'Scuse me for getting up on my soapbox and yelling for so long... IMO