Upward expander - help me understand

[viverivive]

I would like to restore some dynamics in some of my old songs which have been heavily compressed and even limited during the mastering.
I don't have the original mixes, and I know that what is gone is gone, but I still want to do some experiments using an upward expander on those files.

AFAIK, this is basically how an upward expander works: threshold determines the input level at which the expander activates. Negative ratio sets the amount of amplification of the signal that passes the threshold. Knee affects how the expander behaves with signals that are very near the threshold. Attack time is the amount of time the expander will go from zero expansion to full expansion based on the ratio and threshold settings, and release time determines how long it takes for the expander to stop expansion.

And this is the process I'm following:
1) normalize the master file to -6db to leave some space to the expander and avoid clipping;
2) using Izotope Ozone 5, set the threshold just above the average loudness of the song (e.g. -7.5db) with a "gentle" ratio (e.g. 1.5:1.0), with hard knee, slow attack and long release times.
Actually, the magic happens! Well, it does not obviously restore the original mix, but at least it gives back some dynamics to the song.

BUT there is something I can't figure out.
I tried to apply this process on many songs, and in every song the first sound clips. No matter what kind of sound is (drum, bass, guitar, etc.), no matter how I set the knee and attack/release times, no matter the ratio I use, it just clips.
And this seems to have zero sense, since the first sound of every song is under the threshold, and at that point the expander simply shouldn't do anything at all. What happens instead is something like a "turning on engine" sound during the first fractions on silence, and then a boomy clipping sound when the song starts. The trick works just fine for the rest of the song.

What am I missing?
Every help is truly appreciated.

[BertKoor]

So what if you insert a piece of noise before your music to absorb the shock and get it going smoothly?