Vol. 1 i.e. Book of rAma's child hood - bAla kAnDa
Chapter 15 i.e. sarga 15
Verse 32 i.e. SlOka 32
tadaa deva rishi gandharvAH
sa rudrAH sa apsarO gaNAH
stutibhiH divya rUpAbhiH
King daSaratha started the putrakAmEShThi sacrifice, a special sacrifice to beget children.
Gods and sages, accompanied by Creator, prayed Lord VishNu to save them from the evil deeds of the demon rAvaNa.
viShNu promised to get born as a human and annihilate rAvaNa.
The Gods, Sages, Singer-Gods, rudrAs and heaven's nymphs returned praising the glories of Lord VishNu (madhusUdana).
The word 'rudrAs' is very important.
According to current understanding we get from preachers, rudrA is none but Lord Shiva. He is also called mahadEva, mahESwara, paramESwara i.e. the supreme God.
See this verse of rigvEda: (source wikipedia).
bhuvanasya pitaraṃ ghīrbhirābhī
rudraṃ divā vardhayā rudramaktau
bṛhantaṃ ṛṣvamajaraṃ suṣumnaṃ
ṛdhagh ghuvema kavineṣitāsaḥ
Rudra by day, Rudra at night we honour with these our songs, the Universe's Father.
Him great and lofty, blissful, undecaying let us call specially as the Sage impels us.
What we can find:
The rudra, whom the rigvEda called "Universe's father", had been equated with some singers, nymphs, sages, and small Gods.
Though there were said to be EkAdaSa rudras i.e. 11 rudras, all of them were supposed to be the 11 forms of Lord Shiva, just as the dvAdaSa AdityAs i.e. 12 AdityAs were treated as the 12 forms of Lord VishNu.
vAlmiki rAmAyaNa was a vaishNavaite book. vaishNavaite books rarely recognise Shiva as a supreme God. Same way, Saivaite books like Siva purANam do not recognise vishNu as supreme God.
E.g. In the daksha yagnam (daksha's sacrifice) story, God vIrabhadra who was born from a hair of Lord Shiva, hit on the teeth of viShNu and removed them. vishNu and Creator brahma seem to have run for their safety to save themselves from the wrath of vIrabhadra.
You can also see another post of mine at this blog, about Siva (or rudra) not finding a place in agastya's hermitage which rAma visited during his forest exile. Sage agastya had 17 places (not idols. Just fire-pits) for 17 Gods. But there was no place for Siva. Skanda (murugan or kumAraswami or shaNmukha or kArtikEya or subrahmaNya) had a place, but not his father Shiva.