From early in my practice I noticed they had there own type of sensual craving not particularly connected with dense forms.
Jhana is not considered "sensual", including when craved for.
Rūparāgo and arūparāgo
The above appears to be lust for the blissful and equanimous feelings of rupa jhana and formless spheres, such as rapture, happiness, equanimity, spaciousness, etc, such as described in AN 9.36:
Take a mendicant who, quite secluded from sensual pleasures, secluded
from unskillful qualities, enters and remains in the first
absorption.... If they don’t attain the ending of defilements, with
the ending of the five lower fetters they’re reborn spontaneously,
because of their passion (dhammarāgena) and love
(dhammanandiyā) for that meditation. They are extinguished there,
and are not liable to return from that world.
Apart from the above, I do not know of any other sutta explanations.
For a personal opinion, Ajahn Buddhadasa said:
The next defilement, the sixth of the fetters, is desire for the bliss
associated with the various stages of concentration on forms (rupa -
raga). The first three grades of Aryian are still not capable of
giving up attachment to the bliss and tranquillity obtainable by
concentrating deeply on forms, but they will succeed in doing so when
they move up to the last stage, that of the Arahant. The fully
concentrated state has a captivating flavor, which can be described as
a foretaste of Nirvana. Though it differs from real Nirvana, it has
more or less the same flavor. While one is fully concentrated, the
defilements are dormant; but they have not evaporated away entirely,
and will reappear as soon as concentration is lost. As long as they
are dormant, however, the mind is empty, clear, free, and knows the
flavor of real Nirvana. Consequently this state can also become a
cause of attachment.
The seventh subtle defilement is desire for the bliss associated with
full concentration on objects other than forms (arupa - raga). It
resembles the sixth fetter, but is one degree more subtle and
attenuated. Concentration on an object such as space or emptiness
yields a tranquillity and quiescence more profound than concentration
on a form, with the result that one becomes attached to that state. No
Arahant could ever become fascinated by any state of pleasant feeling
whatsoever, regardless of where it originated, because an Arahant is
automatically aware of the impermanence, unsatisfactoriness and
nonselfhood of every state of feeling. Other hermits and mystics
practicing concentration in the forest do not perceive the hidden
danger in these blissful states and so become fascinated by and
attached to the flavor of them just as immature people become attached
to the flavor of sensual objects. For this reason the Buddha used the
same word "desire" for both cases. If you think this subject over and
really come to understand it, you will be full of admiration and
respect for these individuals called Aryians.