Howl. Not silent.

This is a question of dialect and so there is no preferred way to pronounce it. Complicating the issue is that this is not a matter of "either/or." For some dialects, the "h" is slightly pronounced - more than in some regions and less than in others.

Of critical importance is whether to proceed it with "a" or "an". With an aspirated "h" - that is with it more pronounced, "a" is appropriate as in "a hero" or "a human." With a silent "h," "an" would be appropriate as in "an hour."

North American editors typical use "a" regardless of how the author pronounces it. As a slight aspirator, I have a subtle "h," which is not as much as it could be, so when speaking I tend to resort to "an historian."

Not silent. However, sometimes, people use "an" before words like "historical", even though "an" is usually reserved for words that begin with vowels.

As this a question of dialect rather than historical research, it has been removed. You might try asking /r/linguistics if you are interested in which dialects use which pronunciation.

Thank you.